The Geographical Distribution of Animals/Chapter 22

The Mollusca being for the most part marine, it does not enter into the plan of this work to go into much detail as to their distribution. The orders and families will, however, be passed briefly in review, and all terrestrial and fresh-water groups discussed in somewhat more detail; with the object of showing how far their distribution accords with that of the higher animals, and to what extent the anomalies they present can be explained by peculiarities of organisation and habits. If the views advocated in our fifth chapter are correct, the regions there marked out must apply to all classes of animals; and it will be the task of the students of each group, to work out in detail the causes which have led to any special features of distribution. All I can hope to do here, is to show, generally and tentatively, that such a mode of treatment is possible; and that it is not necessary, as it is certainly not convenient or instructive, to have a distinct set of "Regions" established for each class or order in the Animal and Vegetable Kingdoms.

For all the Marine groups I have merely summarised the information contained in Mr. Woodward's Manual of the Mollusca, but in the case of the Land Shells I have consulted the most recent general works, and endeavoured to give an accurate, though doubtless a very incomplete, account of the most interesting facts in their distribution. As their classification is very unsettled, I have followed that of the two latest great works, by Martens and Pfeiffer.

—Open seas of all warm regions. Two species fossil in Tertiary deposits.

—Norway to New Zealand, all tropical and temperate seas and coasts.

—Universal, to Greenland; 2 other genera are fossil, in the Lias and Oolite.

—All seas: 4 other genera are fossil, in Eocene and Miocene deposits.

—All the warmer seas.

—Lias to Chalk in Europe, India and North America.

—Indian and Pacific Oceans; and the fossil species from the Silurian Period to the Tertiary, in all parts of the world.

—Lower Silurian to Lias.

—Upper Silurian to Chalk. Found at 16,000 feet elevation in the Himalayas.

—The Strombidæ, or Wing-shells, inhabit tropical and warm seas from the Mediterranean to New Zealand; most abundant in the Indian and Pacific Oceans. There are nearly 200 fossil species, from the Lias to Miocene and recent deposits.

—All seas, most abundant in the Tropics. Trichotropis is confined to Northern seas; Murex and Fusus are cosmopolitan. There are about 700 fossil species, ranging from the Oolite to the Miocene and recent formations.

—The Buccinidæ, or "Whelks," range over the whole world, but some of the genera are restricted. Buccinum inhabits the north and south temperate seas; Monoceros the West Coast of America; Cassidaria the Mediterranean; Phos, Harpa, Eburna, and Ricinula, are confined to the Pacific; Dolium inhabits the Mediterranean as well as the Pacific. There are about 350 fossil species, mostly from the Eocene and Miocene beds.

—The Cones are universally distributed, but this applies only to the genus Pleurotoma. Conus is tropical and sub-tropical, and Cithara is confined to the Philippine Islands. There are about 460 fossil species, from the Chalk formation to the most recent deposits.

—The Volutes are mostly tropical; but a small species of Mitra is found at Greenland, and a Marginella in the Mediterranean. Cymba is confined to the West Coast of Africa and Portugal. Voluta extends south to Cape Horn. There are about 200 fossil species, from the Chalk and Eocene to recent formations.

—The well-known Cowries are found all over the world, but they are much more abundant in warm regions. One small species extends to Greenland. There are nearly 100 fossil species, from the Chalk to the Miocene and recent formations.

—The Naticidæ, or Sea-snails, though most abundant in the Tropics, are found also in temperate seas, and far into the Arctic regions. Two other genera are fossil; and there are about 300 extinct species, ranging from the Devonian to the Pliocene formations.

—These turreted shells are very widely distributed both in temperate and tropical seas; and most of the genera have also a wide range. There are about 400 extinct species, from so far back as the Lower Silurian to the Pliocene formations.

—These are marine, estuary, or fresh-water shells, of an elongated spiral form; they have a world-wide distribution, but are most abundant in the Tropics. Potamides (41 sp.), is the only fresh-water genus, and is found in the rivers of Africa, India and China, to North Australia and California. Another genus is exclusively fossil, and there are about 800 extinct species, ranging from the Trias to the Eocene and recent formations.

—Fresh-water only: lakes and rivers in warm countries, widely scattered. South Palæarctic and Australian regions, from Spain to New Zealand; South Africa, West Africa, and Madagascar; United States. There are about 50 fossil species, from the Wealden and Eocene to recent formations.

—Universal. Cæcum is found in north temperate seas only. The other genera are mostly tropical, but some species reach Iceland and Greenland. There are near 300 species fossil, ranging from the Neocomian to the Pliocene formations.

—The Littorinidæ are mostly found on the coasts in shallow water; as the common Periwinkle (Littorina littorea). They are of world-wide distribution; but Solarium and Phorus are tropical; while Lacuna, Skenea, and most species of Rissoa are Northern. About 180 species are fossil, ranging from the Permian to the Pliocene formations.

—The Paludinidæ, or River-snails, are all fresh-water, and range over the whole world. Paludina (60 sp.), is confined to the Northern Hemisphere; Ampullaria (136 sp.), is tropical; Amphibola (3 sp.), inhabits New Zealand and the Pacific Islands; Valvata (18 sp.), North America and Britain. There are 72 fossil species of Paludina and Valvata, in the Wealden formation and more recent fresh-water deposits.

—All warm seas, ranging north to Norway and the Caspian Sea. Neritina and Navicella inhabit fresh or brackish waters, the latter confined to the countries bordering the Indian Ocean and the islands of the Pacific. There are 80 fossil species, from the Trias, Lias, and Eocene formations down to recent deposits.

—The genus (200 sp.) has a world-wide range, but the other genera are mostly tropical, and are most abundant in the Indian and Pacific Oceans. There are more than 900 fossil species, found in all parts of the world, from the Lower Silurian to the Tertiary formations.

—The Ear-shells are most abundant in the Indian and Pacific Oceans; some are found on the east coasts of the Atlantic, but there are very few in the West Indies. Ianthina (10 sp.) consists of floating oceanic snails found in the warm parts of the Atlantic. Three other genera are fossil, and there are near 500 fossil species of this family ranging from the Lower Silurian to the Pliocene formations.

—All seas. Puncturella (6 sp.) is confined to Northern and Antarctic seas; Rimula to the Philippines; and Parmophorus (15 sp.) from the Cape of Good Hope to the Philippines and New Zealand. There are about 80 fossil species, ranging from the Carboniferous formation to the deposits of the Glacial epoch.

—The Calyptræidæ, or Bonnet-Limpets, are found on the coasts of all seas from Norway to Chili and Australia; but are most abundant within the Tropics. The genera are all widely scattered. There are 75 fossil species, ranging from the Devonian to recent formations.

—The Patellidæ, or Limpets, are universally distributed, and are as abundant in the temperate as in tropical seas. There are about 100 fossil species, ranging from the Silurian to the Tertiary formations.

—The genus Dentalium is found in the North Atlantic, Mediterranean, West Indies and India. There are 125 fossil species, found in various formations as far back as the Devonian in Europe and in Chili.

—On rocky shores in all parts of the world. There are 37 fossil species ranging back to the Silurian period.

The Land and Fresh-water snails are so important and extensive a group, and their classification has been so carefully studied, that their geographical distribution is a subject of much interest. The range of the genera will therefore be given in some detail. For the Helicidæ I follow the classical work of Albers—Die Helicien, Von Martens' Edition (1860); and for the Operculate families, Pfeiffer's Monographia Pneumonopomorum Viventium, 2nd Supplement, 1865. The number of species is, of course, very considerably increased since these works were published (and the probable amount of the increase I have in most cases indicated), but this does not materially affect the great features of their geographical distribution.

—Universal.

The Helicidæ, or Snails, are a group of immense extent and absolutely cosmopolitan in their range, being found in the most barren deserts and on the smallest islands, all over the globe. They reach to near the line of perpetual snow on mountains, and to the limit of trees or even considerably beyond it, in the Arctic regions; but they are comparatively very scarce in all cold countries. The Antilles, the Philippine Islands, Equatorial America, and the Mediterranean sub-region are especially rich in this family. Comparatively few of the genera, and those generally small ones, are restricted to single regions; but on the other hand very few are generally distributed, only two—Helix and Pupa—occurring in all the six regions, while Helix alone is truly cosmopolitan, occurring in every sub-region, in every country, and perhaps in every island on the globe.

The Neotropical region is, on the whole, the richest in this family, the continental Equatorial districts producing an abundance of large and handsome species, while the Antilles are pre-eminent for the number of their peculiar forms. This region possesses 22 of the genera, and 6 of them are peculiar.

The Palæarctic region seems to come next in productiveness, but this may be partly owing to its having been so thoroughly explored. It possesses 16 of the genera, and 3 of them are confined to it. The great mass of the species are found in the warm and fertile countries surrounding the Mediterranean Sea.

The Ethiopian region has 13 genera, only one of which is peculiar.

The Australian region has 14 genera, 2 of which are confined to the Pacific Islands.

The Oriental has 15 genera and the Nearctic 12, but in neither case are there any peculiar generic types.

The following is the distribution of the several genera taken in the order of their magnitude:—

Helix (1,115 sp.), cosmopolitan. This genus is divided into 88 sub-genera, a number of which have a limited distribution. An immense quantity of species have been recently described, so that the number now exceeds 2,000.

Nanina (290 sp.) is characteristic of the Oriental and Australian regions, over the whole of which it extends, just entering the Palæarctic region as far as North China and Japan. Isolated from this area is a small group of 4 species occurring in West Africa. The number of species in this genus have now been increased to about 400.

Clausilia (272 sp.) is most abundant in Europe, with a few species widely scattered in India, Malaya, China, Japan, Equatorial America, and one in Porto Rico. The described species have been increased to nearly 500.

Bulimulus (210 sp.) is American, and almost exclusively Neotropical, ranging from Montevideo and Chili, to the West Indian Islands, California and Texas; with two sub-genera confined to the Galapagos Islands. About 100 new species have been described since the issue of the second edition of Dr. Woodward's Manual.

Pupa (210 sp.) abounds most in Europe and the Arctic regions, but has a very wide range, being scattered throughout Africa, continental India, Australia, the Pacific Islands, North America to Greenland, and the Antilles; but it is absent from South America, the Himalayan and Malayan sub-regions, China and Japan. An extinct species has occurred abundantly in the carboniferous strata of North America. About 160 additional species have been described.

Bulimus (172 sp.) abounds most in Tropical South America; it is also found from Burmah eastward through Malaya to the Solomon and Fiji Islands; there are also scattered species in Patagonia, St. Vincents, Texas, St. Helena, and New Zealand. More than 100 additional species have been described.

Buliminus (132 sp.) ranges from Central and South Europe over the whole Ethiopian and Oriental regions to North China, and through the Australian to New Zealand; there is also a single outlying species in the Galapagos Islands. About 50 more species have been described.

Cochlostyla (127 sp.) is almost peculiar to the Philippine Islands, beyond which, are a species in Borneo, one in Java, and two in Australia. Very few new species have been added to this genus.

Achatinella (95 sp.) is absolutely confined to the Sandwich Island group. Recent researches have more than tripled the number of described species.

Achatina (87 sp.) is most abundant and finest in the Ethiopian region, over the whole of which it ranges; but there are also species in Florida, the Antilles, the Sandwich Islands, Ceylon and India. The described species are now more than doubled.

Hyalina (84 sp.) inhabits all Tropical America and the Antilles, North America to Greenland, and Europe to the Arctic regions. Comparatively few new species have been described.

Cylindrella (83 sp.) inhabits the West Indian islands and Guatemala to Texas, with a sub-genus in the Philippine Islands. Species since described have more than trebled the number in this genus.

Cionella (67 sp.) is widely scattered; in India from Ceylon to the Khasia Mountains, Brazil, New Granada, the West Indian islands, Palæarctic, and northern part of Nearctic regions, Pacific Islands, New Zealand, and Juan Fernandez. About 20 new species have since been described.

Glandina (66 sp.), Peru to South Carolina and the Antilles, with three species in Central Africa and one in South Europe. About 40 species have been added to this genus.

Stenogyra (49 sp.), widely distributed: Tropical America and West Indies to Florida, South and West Africa, the Mediterranean region, India and the Philippines. About a dozen new species have been described.

Succinea (41 sp.), widely scattered in all the regions, and in St. Helena, Juan Fernandez, Tahiti, Chiloe, Greenland, West Africa, Himalayas and Australia. The described species are now more than 100.

Partula (39 sp.), Solomon Islands to Tahiti and Sandwich Islands. This genus has also been increased to near 100 species.

Streptaxis (34 sp.), most abundant in Tropical South America, but occurs in West Africa, the Seychelles and Rodriguez Islands, Ceylon and Burmah. It now contains over 100 described species.

Spiraxis (33 sp.), Yucatan to Mexico, and less abundant in the West Indian Islands. About 20 species have been added.

Macroceramus (27 sp.), Antilles, Florida, and Peru. The species have been more than doubled.

Vitrina (26 sp.), widely scattered through North and Central Europe, North-west America and Greenland, Abyssinia, Madagascar and South Africa, Himalayas to Burmah and Australia. Species since described have more than doubled the number in this genus.

Orthalicus (23 sp.), Bolivia to Mexico and Antilles. This genus has been increased to about 40 species.

Sagda (19 sp.), Antilles only. Very few new species, if any, have been described.

Zonites (12 sp.), South Europe, with one species of a distinct type in Guatemala. The number of species in this genus has been since about tripled.

Leucochroa (11 sp.), Mediterranean region to Syria and Arabia Petrea.

Simpulopsis (7 sp.), Bahia, Antilles, and far away in the Solomon Islands. Two or three have been added.

Balea (6 sp.), Middle and North Europe, Brazil, and the Island of Tristan d'Acunha.

Daudebardia (6 sp.), Central and South Europe; and a species has since been discovered in New Zealand.

Macrocyclis (4 sp.), Chili, California, Oregon and Central North America.

Columna (3 sp.), West Africa, Princes Islands and Madagascar.

Stenopus (2 sp.), Island of St. Vincent (West Indies.)

Pfeifferia (2 sp.), Philippines and Moluccas.

Testacella (2 sp.), West Europe and Teneriffe. About 8 species have been since described, including one from New Zealand.

Fossil species of Helix, Bulimus, Achatina, Balea, and Clausilia, are found in all the Tertiary formations; while a species of Pupa (as already stated) occurs in the carboniferous formation. For interesting details of the distribution of the sub-genera and species of Achatinella in the Sandwich Islands, see a paper by Rev. J. T. Gulick in the Journal of the Linnean Society. (Zoology, vol. xi. p. 496.)

The Limacidæ, or Slugs, are widely distributed, but they are absent from South America, where they are represented by the next family. They also seem to be absent from the greater part of Africa. The genera are distributed as follows:—

Limax (51 sp.), Palæarctic region, Australia and the Sandwich Islands; Anadenus (2 sp.), Himalayas; Philomycus (9 sp.), North America, China and Java; Arion (25 sp.), Norway to Spain and South Africa; Parmacella (7 sp.), South Europe, Canary Islands and North India; Janella (1 sp.), New Zealand; Aneitea (1 sp.), New Hebrides and New Caledonia; Parmarion (4 sp.), India; Triboniophorus (3 sp.), Australia; Testacella (3 sp.), South Europe, Canary Islands, and New Zealand; Hyalimax (2 sp.), Bourbon and Mauritius; Krynickia (8 sp.), Eastern Europe and North America. A few species of Limax, Arion, and Testacella have been found fossil in Tertiary deposits.

The Oncidiadæ, or Slugs with a coriaceous mantle, inhabit the Oriental region, Mauritius, Australia, the Pacific Islands, South America and South Europe. The genera are:—

Oncidium (16 sp.), South Europe (1 sp. British), Mauritius, Australia and Pacific Islands; Vaginulus (20 sp.), Neotropical and Oriental regions.

The Limnæidæ, or Fresh-water Snails, inhabit ponds and rivers in most parts of the world, but appear to be absent from the Australian region. The genera are distributed as follows:—

Limnæa (95 sp.), Nearctic, Palæarctic, and Oriental regions; Choanomphalus (2 sp.), Lake Baikal; Pompholyx (2 sp.), Western America; Chilinia (18 sp.), South America; Physa (20 sp.), Nearctic, Palæarctic, Ethiopian and Oriental regions, and extends to above 73° North Latitude in Siberia, being the most Arctic of land or fresh-water shells; Ancylus (49 sp.), Nearctic and Neotropical regions, Europe and New Zealand; Planorbis (145 sp.), Nearctic, Palæarctic and Oriental regions. Several genera are found fossil, chiefly in the Wealden, Eocene, and Miocene formations.

The Auriculidæ are chiefly found near the sea in hot countries, and are most abundant in the Eastern tropics. They are absent from the East coast of South America. The genera have a somewhat restricted distribution as follows:—

Auricula (128 sp.), India, Pacific Islands, Peru and West Indies; Melampus (56 sp.), West Indies and Europe; Carychium (9 sp.), Europe and North America; Plectrotrema (14 sp.), Australia, Malay Islands, China, Cuba; Blauneria (2 sp.), West Indian and Sandwich Islands. There are many fossil species ranging back to the Eocene formation.

The Aciculidæ are small cylindrical shells chiefly found in the West Indian Islands, but with representatives widely scattered over the globe.

Acicula (5 sp.) is European only; Geomelania (21 sp.), and Chittya (1 sp.), are confined to the Island of Jamaica; Truncatella (38 sp.), is most abundant in the Antilles, but is also found in some part of each of the six regions, as indicated by the diagram of the family. But few new species have been added to this group.

The Diplommatinidæ are minute shells of the Oriental and Australian regions.

Diplommatina (18 sp.) inhabits India to Burmah, and the greater part of the Australian region; the number of species has now been doubled, and one has been discovered in the island of Trinidad; Clostophis (1 sp.), Moulmein; Paxillus (3 sp.), Borneo, Hong Kong, and Loo Choo Islands.

This extensive group, comprising the largest of the operculated land-shells, is especially characteristic of the Oriental region, which possesses 25 genera, no less than 12 of them being wholly confined to it. The Neotropical region comes next, with 15 genera, 9 of which are peculiar; but a large number of these are confined to the West Indian Islands, South America itself being very poor in this group. The Palæarctic region has 3 peculiar genera; the Ethiopian and Australian 1 each. The Nearctic region has but a single West Indian species in Florida. The distribution of the genera is as follows:—

Peculiar to or characteristic of the Oriental region are, Opisthoporus (11 sp.), Rhiostoma (6 sp.), Alycaeus (39 sp.), Opisthostoma (1 sp.), Hybocystis (3 sp.), Pterocyclos (19 sp.), extending to the Moluccas; Aulopoma (4 sp.), Dermatocera (4 sp.), Leptopoma (54 sp.), extending west to the Seychelles and east to the Moluccas and New Guinea; Cyclophorus (163 sp.), most abundant in the Oriental region, but ranges to Japan, to Chili, and all Tropical America, over the whole Australian region, and to Natal and Madagascar; Cataulus (15 sp.), confined to Ceylon, the Neilgherries and Nicobar Islands; Rhaphaulus (4 sp.), Penang to Ceram; Streptaulus (1 sp.), Arinia (3 sp.), Pupinella (2 sp.), Pupina (24 sp.), half in North India to Philippines and Japan, the other half in Moluccas, New Guinea and Australia; Cyclotopsis (2 sp.), India and Malaya; Registoma (9 sp.), Philippines and Moluccas, New Caledonia and Pacific.

Characteristic of the Neotropical region are:—Cyclotus (111 sp.), half in the Antilles and Tropical America, the rest in the Moluccas, China, Malaya, India, Natal, and the Seychelle Islands; Megalomastoma (27 sp.), abundant in Cuba, West Indies and South America, others in India, Malaya, and Mauritius; Jamaicia (2 sp.), Jamaica; Licina (5 sp.), Antilles; Choanopoma (49 sp.), Antilles; Ctenopoma (25 sp.), Antilles; Diplopoma (1 sp.), Cuba; Adamsiella (15 sp.), Jamaica, Cuba, Guatemala; Cyclostomus (113 sp.), abundant in Antilles, also occurs in Madagascar, Arabia, Syria, Hungary and New Zealand; Tudora (34 sp.), Antilles, and one species in Algeria; Cistula (40 sp.), Chondropoma (94 sp.), Bourcieria (2 sp.), Tropical America.

Peculiar to or characteristic of the Palæarctic region are:—Craspedopoma (5 sp.), confined to Madeira, the Azores and Canaries; Leonia (1 sp.), Spain and Algeria; Pomatias (22 sp.), Europe and Canaries with a species in the Himalayas; Cecina (1 sp.), Manchuria.

The Ethiopian region has the peculiar genus Lithidion (5 sp.), Madagascar, Socotra and Arabia; and Otopoma (19 sp.), Mascarene Islands and Socotra, with a species in Western India and another in New Ireland.

The Australian region is characterised by Callia (3 sp.), in Ceram, Australia, and the Philippines respectively; Realia (7 sp.), New Zealand and the Marquesas Islands; Omphalotropis (38 sp.), the Australian region, with some species in India, Malaya, and the Mauritius.

The remaining genus, Hydrocena (27 sp.), has a very widely scattered distribution, being found in South Europe, Japan, the Cape, China, Malaya, New Zealand, the Pacific Islands and Chili. From 10 to 20 per cent. of new species have been since described in most of the genera of this family.

The Helicinidæ are very characteristic of the Antilles, comparatively few being found in any other part of the world except the Islands of the Pacific. The genera are:—

Trochatella (33 sp.), Antilles with a species in Venezuela, and another in Cambodja; Lucidella (5 sp.), Antilles; Helicina (274 sp.), Antilles, Pacific Islands, Tropical America, Southern United States, Moluccas, Australia, Philippines, Java, Andaman Islands and North China; Schasicheila (5 sp.), Mexico, Guatemala and Bahamas; Alcadia (28 sp.), Antilles; Georissa (5 sp.) Moulmein to Burmah. About 10 per cent. of new species appear to have been since described in the larger genera of this family.

A consideration of the distribution of the families and genera of land-shells shows us, that although they possess some special features, yet they agree in many respects with the higher animals in their limitation by great natural barriers, such as oceans, deserts, mountain ranges, and climatal zones. A remarkable point in the distribution of these animals, is the number of genera which have a very limited range, and also the prevalence of genera having species scattered, as it were at random, all over the earth. No less than 14 genera (or about one-sixth of the whole number) are confined to the Antilles, while the greater part of the sub-genera of modern authors are restricted to limited areas.

If we first compare the New World with the Old, we find the difference as regards genera quite as great as in most of the vertebrates. In the Helicidæ, 10 genera are confined to the New, and 7 to the Old World, 16 being common to both. In the Operculata the number of genera of restricted range is greater,—the New World having 15, the Old World 32 genera, only 8 being common to both. Of the New World genera 12 out of the 15 do not occur at all in South America; and of those of the Old World, 22 out of the 32 occur in a single region only. If we take the northern and southern division proposed by Professor Huxley (the latter comprising the Australian and Neotropical regions), we find a much less well-marked diversity. Among the Helicidæ only 4 are exclusively northern, 8 southern; while among the Operculata 22 are northern, 16 southern. The best way to compare these two kinds of primary division will be to leave out all those genera confined to a single region each, and to take account only of those characteristic of two or more of the combined regions; which will evidently show which division is the most natural one for this group. The result is as follows:—

We find then that the northern and southern division of the globe is not at all supported by the distribution of the terrestrial molluscs. It is indeed very remarkable, that the connection so apparent in many groups between Australia and South America is so scantily indicated here. The only facts supporting it seem to be, the occurrence of Geotrochus (a sub-genus of Helix) in Brazil, as well as in the Austro-Malayan and West Pacific Islands and North Australia; and of Bulimus in the same two parts of the globe, but peculiar sub-genera in each. But in neither case is there any affinity shown between the temperate portions of the two regions, so that we must probably trace this resemblance to some more ancient diffusion of types than that which led to the similarity of plants and insects. Still more curious is the entire absence of genera confined to, and characteristic of Africa and India. One small sub-genus of Helix, (Rachis), and one of Achatina, (Homorus), appear to have this distribution,—a fact of but little significance when we find another sub-genus of Helix, (Hapalus), common and confined to Guinea and the Philippine Islands; and when we consider the many other cases of scattered distribution which cannot be held to indicate any real connection between the countries implicated. No genus is confined to the Palæarctic and Nearctic regions as a whole. A large number of sub-genera, many of them of considerable extent, are peculiar to one or other of these regions, but only 3 sub-genera of Helix and 2 of Pupa are common and peculiar to the two combined, and these are always such as have an Arctic range and whose distribution therefore offers no difficulty.

We find, then, that each of our six regions and almost all of our sub-regions are distinctly confirmed by the distribution of the terrestrial mollusca; while the different combinations of them which have at various times been suggested, receive little or no support whatever. Even those remarkably isolated sub-regions, New Zealand and Madagascar, have no strictly peculiar genera of land-shells, although they both possess several peculiar sub-genera; being thus inferior in isolation to some single West Indian Islands, to the Sandwich Islands, and even to the North Atlantic Islands (Canaries, Madeira, and Azores), each of which have peculiar genera. This of course, only indicates that the means by which land mollusca have been dispersed are somewhat special and peculiar. To determine in what this speciality consists we must consider some of the features of the specific distribution of this group.

The range of genera, and even of sub-genera is, as we have seen, often wide and erratic, but as a general rule the species have a very restricted area.

Hardly a small island on the globe but has some land-shells peculiar to it. Juan Fernandez has 20 species, all peculiar. Madeira and Porto Santo have 109 peculiar species out of a total of 134. Every little valley, plain, or hill-top, in the Sandwich Islands, though only a few square miles in extent, has its peculiar species of Achatinella. Another striking feature of the distribution of land molluscs, is the richness of islands as compared with continents. The Philippines contain more species than all India; and those of the Antilles according to Mr. Bland almost exactly equal the numbers found in the entire American continent from Greenland to Patagonia. Taking the whole world, it appears that many more species of land-shells are found in the islands than on the continents of the globe, a peculiarity that obtains in no other extensive group of animals.

Looking at these facts it seems probable, that the air-breathing molluscs have been chiefly distributed by air- or water-carriage, rather than by voluntary dispersal on the land. Even seas and oceans have not formed impassable barriers to their diffusion; whereas they only spread on dry land with excessive slowness and difficulty. The exact mode in which their diffusion is effected is not known, and it may depend on rare and exceptional circumstances; but it seems likely to occur in two ways. Snails frequently conceal themselves in crevices of trees or under bark, or attach themselves to stems or foliage, and either by their operculum or mucous diaphragm, are able to protect themselves from the injurious effects of salt water for long periods. They might therefore, under favourable conditions, be drifted across arms of the sea or from island to island; while wherever there are large rivers and occasional floods, they would by similar means be widely scattered over land areas. Another possible mode of distribution is by means of storms and hurricanes, which would carry the smaller species for long distances, and might occasionally transport the eggs of the larger forms. Aquatic birds might occasionally get both shells and eggs attached to their feet or their plumage, and convey them across a wide extent of sea. But whether these, or some other unknown agency has acted, the facts of distribution clearly imply that some means of transport over water is, and has been, the chief agent in the distribution of these animals; but that its action is very rare or intermittent, so that its effects are hardly perceptible in the distribution of single species.

Another important factor in enabling us to account for the distribution of these animals is the geological antiquity of the group, and the amount of change exhibited in time, by species and genera. Now we find that most of the genera of land-shells range back to the Eocene period, while those inhabiting fresh water are found almost unchanged in the Wealden. In North America a species of Pupa and one of Zonites, have been discovered in the coal measures, along with Labyrinthodonts; and this fact seems to imply, that many more terrestrial molluscs would be discovered, if fresh-water deposits, made under favourable conditions, were more frequently met with in the older rocks. If then the existing groups of land-molluscs are of such vast antiquity, and possess some means, however rarely occurring, of crossing seas and oceans, we need not wonder at the wide and erratic distribution now presented by so many of the groups; and we must not expect them to conform very closely to those regions which limit the range of animals of higher organization and less antiquity.

The total number of species of pulmoniferous mollusca is about 7,000, according to the estimate of Mr. Woodward, brought down to 1868 by Mr. Tate. But this number would be largely increased if the estimates of specialists were taken. Mr. Woodward for example, gives 760 as the number of species in the West Indian Islands; whereas Mr. Thomas Bland, who has made the shells of these islands a special study, considers that there were 1,340 species in 1866. So, the land-shells of the Sandwich Islands are given at 267; but Mr. Gulick has added 120 species of Achatinellidæ, bringing the numbers up to nearly 400,—but no doubt several of these are so closely related that many conchologists would class them as varieties. The land-shell fauna of the Antilles is undoubtedly the most remarkable in the world, and it has been made the subject of much interesting discussion by Mr. Bland and others. This fauna differs from that of all other parts of the globe in the proportions of the operculate to the inoperculate shells. The Operculata of the globe are about one-seventh, the Inoperculata about six-sevenths of the whole; and some general approximation to this proportion (or a much smaller one) exists in almost all the continents, islands, and archipelagoes. In the Philippines, for example, the proportion of the Operculata is a little more than one-seventh; in the Mauritius, between one-third and one-fourth; in Madeira, one-fourteenth; in the whole American continent about one-eighth; but when we come to the Antilles we find them to amount to nearly five-sixths, about half the Operculata of the globe being found there!

Mr. Bland endeavours to ascertain the source of some of the chief genera found in the West Indian Islands, on the principle that "each genus has had its origin where the greatest number of species is found;" and then proceeds to determine that some have had an African, some an Asiatic, and some an American origin, while others are truly indigenous. But we fear there is no such simple way of arriving at so important a result; and in the case of groups of extreme antiquity like the genera of mollusca, it would seem quite as possible that the origin of a genus is generally not where the greatest number of species are now found. For during the repeated changes of physical conditions that have everywhere occurred since the Eocene period (to go no further back) every genus must have made extensive migrations, and have often become largely developed in some other district than that in which it first appeared. As a proof of this, we not unfrequently find fossil shells where the species and even the genus now no longer exists; as Auricula, found fossil in Europe, but only living in the Malay and Pacific Islands; Anastoma and Megaspira, now peculiar to Brazil, but fossil in the Eocene of France; and Proserpina of the West Indies, found in the Eocene formation of the Isle of Wight. The only means by which the origin of a genus can satisfactorily be arrived at, is by tracing back its fossil remains step by step to an earlier form; and this we have at present no means of doing in the case of the land-shells. Taking existing species as our guide we should certainly have imagined that the genus Equus originated in Africa or Central Asia; but recent discoveries of numerous extinct species and of less specialized forms of the same type, seem to indicate that it originated in North America, and that the whole tribe of "horses" may be, for anything we yet know to the contrary, recent immigrants into the Old World! This example alone must convince us, that it is impossible to form any conclusion as to the origin of a genus, from the distribution of existing species only.

The general conclusion we arrive at, therefore, is, that the causes that have led to the existing distribution of the genera and higher groups of the terrestrial mollusca are so complex, and have acted through such long periods, that most of the barriers which limit the range of other terrestrial animals do not apply to them, although the species are, in most cases, strictly limited by them. Some means of diffusion—which, though probably acting very slowly and at long intervals, and more powerfully on continents than between islands, is yet highly efficient when we consider the long duration of genera—has, to a considerable extent, dispersed them across continents, seas, and oceans. On the other hand, those mountain barriers which separate many groups of the higher vertebrates, are generally less ancient than the genera of land-shells, which are thus often distributed independently of them. In order to compare the distribution of the terrestrial mollusca on equal terms with those of land animals generally, we must take genera of the former as equivalent to family groups of the latter; and we shall, I believe, then find that the distribution of the sub-genera and smaller groups of species do accord mainly with those divisions of the earth into regions and sub-regions which we have here indicated. Mr. Harper Pease, in a communication on Polynesian Land Shells in the Proceedings of the Zoological Society for 1871 (p. 449), marks out the limits of the Polynesian sub-region, so as exactly to agree with that arrived at here from a consideration of the distribution of vertebrata; and he says that this sub-region, (or region, as he terms it) is distinctly characterised by its land-shells from all the surrounding regions. The genera (or sub-genera) Partula, Pitys, Achatinella, Palaina, Omphalotropis, and many others, are either wholly confined to this sub-region or highly characteristic of it. Mr. Binney, in his Catalogue of the Air-breathing Molluscs of North America, marks out our Nearctic region (with almost identical limits) as most clearly characterised. He also arrives at a series of sub-divisions, which generally (though not exactly) agree with the sub-regions which I have here adopted. The Palæarctic, the Ethiopian, and the Oriental regions, are also generally admitted to be well characterised by their terrestrial molluscs. There only remain the Australian and the Neotropical regions, in which some want of homogeneity is apparent, owing to the vast development and specialisation of certain groups in the islands which belong to these regions. The Antilles, on the one hand, and the Polynesian Islands, on the other, are so rich in land-shells and possess so many peculiar forms, that, judged by these alone, they must form primary instead of secondary divisions. We have, however, already pointed out the inconvenience of any such partial systems of zoological geography, and the causes have been sufficiently indicated which have, in the case of land-shells as of insects, produced certain special features of distribution.

We therefore venture to hope, that conchologists will give us the advantage of their more full and accurate knowledge both of the classification and distribution of this interesting group of animals, not to map out new sets of regions for themselves, but to show what kind of barriers have been most efficient in limiting the range of species, and how their distribution is actually effected, so as to be able to explain whatever discrepancies exist between the actual distribution of land-shells and that of the higher animals.

There are ten families in this order, all of which, as far as known, are widely or universally distributed. Some of them are found fossil, ranging back to the Carboniferous epoch. They are commonly termed Sea-slugs, and have either a thin small shell or none. We shall therefore simply enumerate the families, with the number of genera and species as given by Mr. Woodward.

31.—TORNATELLIDÆ. (7 Genera, 62 Species living, 166 fossil.)

32.—BULLIDÆ. (12 Genera, 168 Species living, 88 fossil.)

33.—APHYSIADÆ. (8 Genera, 84 Species living, 4 fossil.)

34.—PLEUROBRANCHIDÆ. (7 Genera, 28 Species living, 5 fossil.)

35.—PHYLLIDIADÆ. (4 Genera, 14 Species living, 0 fossil.)

36.—DORIDÆ. (23 Genera, 160 Species living, 0 fossil.)

37.—TRITONIADÆ. (9 Genera, 38 Species living, 0 fossil.)

38.—ÆOLIDÆ. (14 Genera, 101 Species living, 0 fossil.)

39.—PHYLLYRHOIDÆ. (1 Genus, 6 Species living, 0 fossil.)

40.—ELYSIADÆ. (5 Genera, 13 Species living, 0 fossil.)

These are oceanic, swimming molluscs, of a delicate texture. They are found in all warm seas, and range back to the Lower Silurian epoch. There are only two families.

41.—FIROLIDÆ. (2 Genera, 33 Species living, 1 fossil.)

42.—ATLANTIDÆ. (5 Genera, 22 Species living, 159 fossil.)

These are swimming, oceanic mollusca, inhabiting both Arctic, Temperate, and Tropical seas. The three families have each a wide distribution in all the great oceans. They range back to the Silurian period.

1.—HYALEIDÆ. (9 Genera, 52 Species living, 95 fossil.)

2.—LIMACINIDÆ. (4 Genera, 19 Species living, 0 fossil.)

3.—CLIONIDÆ. (4 Genera, 14 Species living, 0 fossil.)

These are sedentary, bivalve, marine mollusca, having laterally symmetrical shells, but with unequal valves. Both in space and time they are the most widely distributed molluscs. They are found in all seas, and at all depths; and when any of the families or genera have a restricted range, it seems to be due to our imperfect knowledge, rather than to any real geographical limitations. In time they range back to the Cambrian formation, and seem to have had their maximum development in the Silurian period. It is not, therefore, necessary for our purpose, to do more than give the names of the families with the numbers of the genera and species, as before.

1.—TEREBRATULIDÆ. (5 Genera, 67 Species living, 340 fossil.)

2.—SPIRIFERIDÆ. (4 Genera, 0 Species living, 380 fossil.)

3.—RHYNCHONELLIDÆ. (3 Genera, 4 Species living, 422 fossil.)

4.—ORTHIDÆ. (4 Genera, 0 Species living, 328 fossil.)

5.—PRODUCTIDÆ. (3 Genera, 0 Species living, 146 fossil.)

6.—CRANIADÆ. (1 Genus, 5 Species living, 37 fossil.)

7.—DISCINIDÆ. (2 Genera, 10 Species living, 90 fossil.)

8.—LINGULIDÆ. (2 Genera, 16 Species living, 99 fossil.)

The Conchifera, or ordinary Bivalve Molluscs, may be distinguished from the Brachiopoda by having their shells laterally unsymmetrical, while the valves are generally (but not always) equal. They are mostly marine, but a few inhabit fresh water. As the distribution of some of the families presents points of interest, we shall treat them in the same manner as the marine Gasteropoda.

—The Ostreidæ, including the Oysters and Scallops, are found in all seas, Arctic as well as Tropical. There are nearly 1,400 species fossil, ranging back to the Carboniferous period.

—The Aviculidæ, or Wing-shells and Pearl Oysters, are characteristic of Tropical and warm seas, a few only ranging into temperate regions. Nearly 700 fossil species are known from various formations ranging back to the Devonian, and Lower Silurian.

—The Mytilidæ, or Mussels, have a world-wide distribution. There is one fresh-water species, which inhabits the Volga. There are about 350 fossil species, ranging back to the Carboniferous epoch.

—The Arcadæ are universally distributed, and are most abundant in warm seas. The genus Leda is, however, abundant in Arctic and Temperate regions, and Solenella is confined to the South Temperate zone. There are near 1,200 fossil species, found in all strata as low as the Lower Silurian.

—The living Trigoniæ are confined to Australia, but there are 5 other genera fossil, containing about 150 species, and found in various formations from the Chalk to the Lower Silurian.

—The Unionidæ, or Fresh-water Mussels, are found in all the fresh waters of the globe, but some of the genera are restricted. Castalia, Mycetopus and Mulleria are confined to the rivers of South America; Anodon, to the Nearctic and Palæarctic regions; Iridina, and Etheria, to the rivers of Africa; Unio has a universal distribution, but is especially abundant in North America. About 60 fossil species are found in the Tertiary and Wealden formations.

—The Chamidæ, or Giant Clams, are confined to Tropical seas, chiefly among coral reefs. There are two other genera and 62 species fossil, ranging from the Chalk to the Oolite formations.

Fossils of doubtful affinity, from the Chalk formation.

—The Tridacnidæ, or Clam-shells, are of very large size, and are confined to the Tropical regions of the Indian and Pacific Oceans. A few species have been found fossil in the Miocene formation.

—The Cardiadæ, or Cockles, are of world-wide distribution. Another genus is fossil, and nearly 400 fossil species are known, ranging back to the Upper Silurian formation.

—The Lucinidæ inhabit the Tropical and Temperate seas of all parts of the world; but the genus Corbis is confined to the Indian and Pacific Oceans, Montacuta and Lepton, to the Atlantic. There are nearly 500 extinct species, ranging from the Tertiary back to the Silurian formation.

—The Cycladidæ are small fresh- or brackish-water shells found all over the globe. The genus Cyclas is most abundant in the North Temperate zone, while Cyrena inhabits the warmer shores of the Atlantic and Pacific, but is absent from the West Coast of America. There are about 150 species fossil, ranging back from the Pliocene to the Wealden formations.

—Universal. Cyprina and Astarte are Arctic and North Temperate; Cardita is Tropical and South Temperate. There are several extinct genera and about 1,000 species found in all formations as far back as the Lower Silurian.

.—Universal. Lucinopsis is confined to the North Atlantic; Glauconeza to the months of rivers in the Oriental region; Meroe and Trigona to warm seas. There are about 350 fossil species, ranging back to the Oolitic period.

—All seas, but more abundant in the Tropics. Gnathodon is found in the Gulf of Mexico; Anatinella in the Oriental region. There are about 60 fossil species, ranging back to the Carboniferous period.

—All seas; most abundant in the Tropics. Galatea is confined to African rivers. There are about 60 fossil species, mostly Tertiary, but ranging back to the Carboniferous period.

—All Temperate and Tropical seas. There are 80 fossil species which range back to the Carboniferous epoch.

—All seas. Panopæa inhabits both North and South Temperate seas; Glycimeris, Arctic seas. There are near 350 fossil species, ranging back to the Lower Oolite formation.

—All seas. Pholadomya is from Tropical Africa; Myadora from the Western Pacific; Myochama and Chamostrea are Australian. There are about 400 fossil species, ranging back to the Lower Silurian formation.

—Temperate and warm seas. Aspergillum ranges from the Red Sea to New Zealand. There are 35 fossil species, ranging back to the Lower Oolite.

—These burrowing molluscs inhabit all Temperate and warm seas from Norway to New Zealand. There are about 50 fossil species, ranging back to the epoch of the Lias.

The marine Mollusca are remarkable for their usually wide distribution. About 48 of the families are cosmopolitan, ranging over both hemispheres, and in cold as well as warm seas. About 15 are restricted to the warmer seas of the globe; but several of these extend from Norway to New Zealand, a distribution which may be called universal, and only 2 or 3 are absolutely confined to Tropical seas. Two small families only, are confined to the Pacific and Indian Oceans. Marine fishes, on the other hand, have a much less cosmopolitan character, no less than 30 families having a limited distribution, while 50 are universal. Some of these 30 families are confined to the Northern seas, some to the Atlantic and Mediterranean, and a considerable number to the Indian Ocean and Western Pacific. Many of these families, it is true, are much smaller than those of the Mollusca, which seem to possess very few of those small isolated families of two or three species only, which abound in all the Vertebrate classes. These differences are no doubt connected with the higher organisation of fishes, which renders them more susceptible to changed conditions of life; and this is indicated by the much less antiquity of existing families of fishes, the greater part of which do not date back beyond the Cretaceous epoch, and many of them only to the Eocene. In striking contrast we have the vast antiquity of most of the families of Mollusca, as shown in the following table of their range taken from Mr. Woodward's work, but re-arranged, and somewhat modified.

Nor is this enormous antiquity confined to family types alone. Many genera are equally ancient. The genus Lingula has existed from the earliest Palæozoic times down to the present day; while Terebratula, Rhynchonella, Discina, Nautilus, Natica, Pleurotomaria, Patella, Dentalium, Mytilus and many other living forms, range back to the Palæozoic epoch. That groups of such immense antiquity, and having power to resist such vast changes of external conditions as they must have been subject to, should now be widely distributed, is no more than might reasonably be expected. It is only in the case of sub-genera and species, that we can expect the influence of recent geological or climatal changes to be manifest; and it must be left to special students to work out the details of their distribution, with reference to the general principles found to obtain among the more highly organised animals.