Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/694

676 676 ICHTHYOLOGY [DISTRIBUTION. of the mountaTus of Asia. Minor, and of the Hindu Rush, they spread to the extreme south of this region. At the present day they are most numerously represented in its northern temperate parts ; towards the south they become fewer, but increase again in numbers and species where- ever a great elevation offers them the snow-fed waters which they affect. In the rivers of the Mediterranean, Salmonoids are by no means rare, but they prefer the upper courses of those rivers, and do not migrate to the sea, with the exception, perhaps, of some species in the rivers of the North Adriatic. The pike, Umbra, and several species of perch and stickle back are also clearly autochthonous species of this region. Others belong to marine types, and seem to have been re- j tained in fresh water at various epochs, as the freshwater Cottas (miller s thumb) ; Coitus quadricortiis, which in- , habits lakes of Scandinavia, whilst other individuals of , the same species are strictly marine ; the burbot (Lota j vulgaris) ; and the singular Comephoriis, a dwartect and much changed Gadoid which inhabits the greatest depths. of Lake Baikal. Remnants of the Palaeichthyic fauna exist in the stur- j geons and lampreys. The former inhabit in abundance the great rivers of eastern Europe and Asia, periodically ascending them from the sea; their southernmost limits are the Yang-tse-Keang in the east, and towards the centre of this region the rivers flowing into the Adriatic, Black, and Caspian Seas, and Lake Aral. None are known to have gone beyond the boundaries of the northern zone. If the lampreys are justly reckoned among freshwater fishes, their distribution is unique and exceptional. In the Palsearctic region some of the species descend periodically to the sea, whilst others remain stationary in the rivers ; the same has been observed in the lampreys of North America. They are entirely absent in the equatorial zone, but reappear in the temperate zone of the southern hemi sphere. Many points in the organization of the Cyclo- stomes indicate that they are a type of great antiquity. The remaining Pahearctic fishes are clearly immigrants from neighbouring regions: thus Silurus, Macrones, and Pseudobagrus have migrated from the Indian region, Ami urns and, as mentioned above, Catostomus from North Am rica. The Cyprinodonts are restricted to the southern and warmer parts, and all belong to the carni vorous division. The facility with which these fishes accommodate themselves to a sojourn in fresh, brackish, or salt water, and even in thermal springs, renders their general distribution easily comprehensible, but it is impos sible to decide to which region they originally belonged ; their remains in Tertiary deposits round the Mediterranean are not rare. The boundaries of the North American or Nearctic Region have been sufficiently indicated. The main features and the distribution of this fauna are identical with those of the preceding region. Out of the 40 families of fresh water fishes 19 are found in this region. The proportion of Cyprinoid species to the total number of North Ameri can fishes (135 : 339) appears to be considerably less than in the Palsearctic region, but we cannot admit that these | figures approach the truth, as the Cyprinoids of North i America have been much less studied than those of Europe; of many scarcely more than the name is known. This also , applies in a great measure to the Salmonoids, of which only | half as many as are found in the Palarsarctic region have been sufficiently described to be worthy of consideration. North America will, without doubt, in the end show as many distinct races as Europe and Asia. Cyprinoids belonging to living as well as extinct genera existed in North America in the Tertiary period. At present the Cyprinina, Leuciscina, and Abramidina are well represented, but there is no representative of the Old World genus Barbus, or of the Cobitidina ; l Rhodeina are also absent. On the other hand, a well marked Cypri noid type is developed : the Catostomina, of which one species has, as it were, returned into Asia. Very charac teristic is the group of Centrarchina, allied to the perch, of which there are some thirty species ; there are two Grystina. Of the sticklebacks there are as many species as in Europe, and of pike not less than seven species have been distinguished. Umbra appears to be as local as in Europe. Some very remarkable forms, types of distinct families, though represented by one or two species only, complete the number of North American autochthonous fishes, viz., Aphredoderus, Percopsis, Hyodon, and the Heteropygii (Amblyopsis and Ckologaster}. The last ar.3 allied to the Cyprinodonts, differing from them in some points of the structure of their intestines. The two genera are extremely similar, but Chologaster, which is found in ditches in the rice-fields of South Carolina, is provided with eyes, and wants the ventral fins. Amblyopsis is the cele brated blind fish of the Mammoth Cave of Kentucky, it is colourless and eyeless, and has rudimentary ventral fins, which occasionally may be entirely absent. A peculiar feature of the North American fish fauna is that it has retained, besides the sturgeons and lampreys, representatives of two Ganoid families, Lepidosteus aad Amia. Both these genera occur in Tertiary formations ; whilst the former is represented in Europe as well as in North America, fossil remains of Amia have been found in the western hemisphere only. It is difficult to account for the presence of the Amiu* rina in North America. They form a well-marked division of the Bagrina, which are well represented in Africa and the East Indies, but are absent in South America ; it is evident, therefore, that they should not be regarded as immigrants from the south, as is the case with the Paliearctic Siluroids. Nor again, has the connexion between South and North America been established sufficiently long to admit of the supposition that these Siluroids could have spread in the interval from the south to the northern parts of the conti nent, for some of the species are found as far north as Pine Islands Lake (54 N. lat.). 2 SOUTHERN ZONE. The boundaries of this zone have been indicated in the description of the equatorial zone ; they overlap the southern boundaries of the latter in South Australia and South America, but we have not at present the means of exactly defining the limits to which southern types extend northwards. This zone includes Tasmania, with at least a portion of south-eastern Australia (Tasmanian sub- region), New Zealand and the Auckland Islands (New Zealand sub-region), and Chili, Patagonia, Tierra del Fuego, and the Falkland Islands (Fuegian sub-region). No fresh water fishes are known from Kerguelen s Land, or from islands beyond 55 S. lat. The southern extremity of Africa has to be excluded from this zone so far as freshwater fishes are concerned. With regard to its extent as well as to the number of species, this zone is the smallest of the three, the number of species known being 11 in the Tasmanian, 8 in the New Zealand, and 18 in the Fuegian sub-region. Yet the ichthyologies! features of this zone are well marked ; they consist in the presence of two peculiar families, each of which is analogous to a northern type, viz., the Haplochi- tonid(K, which represent the Salmonidnu (Haplochiton being 1 Cope has discovered in a Tertiary freshwater deposit at Idaho an extinct genus of this group, Diastichns. He considers this interesting fact to be strongly suggestive of continuity of territory between Asia and North America. Proc. Am. Phil. Soc., 1873, p. 55. 2 Leidy describes a Siluroid (Pimelodus) from the Tertiary deposits of Wyoming territory. Contrib, to the Extinct Vert. Fauna of the Western Territ,, 1873, p. 193.