Page:EB1911 - Volume 08.djvu/143

 a brachiopod. The tabulate corals were represented by Favosites, Michelinia, Pleurodictyum, Fistulipora, Pachypora and others. Heliolites and Plasmopora represent the alcyonarians. Stromatoporoids were important reef builders. A well-known fossil is Receptaculites, a genus to which it has been difficult to assign a definite place; it has been thought to be a sponge, it may be a calcareous alga, or a curious representative of the foraminifera.

In the Devonian period the brachiopods reached the climax of their development: they compose three-quarters of the known fauna, and more than 1100 species have been described. Changes were taking place from the beginning of the period in the relative importance of genera; several Silurian forms dropped out, and new types were coming in. A noticeable feature was the development of broad-winged shells in the genus Spirifer, other spiriferids were Ambocoelia, Uncites, Verneuilia. Orthids and pentamerids were waning in importance, while the productids (Productella, Chonetes, Strophalosia) were increasing. The strophomenids were still flourishing, represented by the genera Leptaena, Stropheodonta, Kayserella, and others. The ancient Lingula, along with Crania and Orbiculoidea, occur among the inarticulate forms. Another long-lived and wide-ranging species is Atrypa reticularis. The athyrids were very numerous (Athyris, Retzia, Merista, Meristella, Kayserina, &c.); and the rhynchonellids were well represented by Pugnax, Hypothyris, and several other genera. The important group of terebratulids appears in this system; amongst them Stringocephalus is an eminently characteristic Devonian brachiopod; others are Dielasma, Cryptonella, Rensselaeria and Oriskania.

The pelecypod molluscs were represented by Pterinea, abundant in the lower members along with other large-winged forms, and by Cucullella, Buchiola and Curtonotus in the upper members of the system. Other genera are Actinodesma, Cardiola, Nucula, Megalodon, Aviculopecten, &c. Gasteropods were becoming more important, but the simple capulid forms prevailed: Platyceras (Capulus), Straparollus, Pleurotomaria, Murchisonia, Macrocheilina, Euomphalus. Among the pteropods, Tentaculites was very abundant in some quarters; others were Conularia and Styliolina. In the Devonian period the cephalopods began to make a distinct advance in numbers, and in development. The goniatites appear with the genera Anarcestes, Agoniatites, Tornoceras, Bactrites and others; and in the upper strata the clymenoids, forerunners of the later ammonoids, began to take definite shape. While several new nautiloids (Homaloceras, Ryticeras, &c.) made their appearance several of the older genera still lived on (Orthoceras, Poterioceras, Actinoceras).

Crinoids were very abundant in some parts of the Devonian sea, though they were relatively scarce in others; they include the genera Melocrinus, Haplocrinus, Cupressocrinus, Calceocrinus and Eleuthrocrinus. The cystideans were falling off (Proteocystis, Tiaracrinus), but blastoids were in the ascendant (Nucleocrinus, Codaster, &c.). Both brittle-stars, Ophiura, Palaeophiura, Eugaster, and true starfishes, Palaeaster, Aspidosoma, were present, as well as urchins (Lepidocentrus).

When we turn to the crustaceans we have to deal with two distinct assemblages, one purely marine, trilobitic, the other mainly lacustrine or lagoonal with a eurypteridian facies. The trilobites had already begun to decline in importance, and as happens not infrequently with degenerating races of beasts and men, they began to develop strange eccentricities of ornamentation in some of their genera. A number of Silurian genera lived on into the Devonian period, and some gradually developed into new and distinctive forms; such were Proëtus, Harpes, Cheirurus, Bronteus and others. Distinct species of Phacops mark the Lower and Upper Devonian respectively, while the genus Dalmania (Odontochile) was represented by species with an almost world-wide range. The Ostracod Entomis (Cypridina) was extremely abundant in places—Cypridinen-Schiefer—while the true Cypridina was also present along with Beyrichia, Leperditia, &c. The Phyllocarids, Echinocaris, Eleuthrocaris, Tropidocaris, are common in the United States. It is in the Old Red Sandstone that the eurypterids are best preserved; foremost among these was Pterygotus; P. anglicus has been found in Scotland with a length of nearly 6 ft.; Eurypterus, Slimonia, Stylonurus were other genera.

Insects appear well developed, including both orthopterous and neuropterous forms, in the New Brunswick rocks. Mr Scudder believed he had obtained a specimen of Orthoptera in which a stridulating organ was present. A species of Ephemera, allied to the modern may-fly, had a spread of wing extending to 5 in. In the Scottish Old Red Sandstone myriapods, Kampecaris and Archidesmus, have been described; they are somewhat simpler than more recent forms, each segment being separate, and supplied with only one pair of walking legs. Spiders and scorpions also lived upon the land.

The great number of fish remains in the Devonian and Old Red strata, coupled with the truly remarkable characters possessed by some of the forms, has caused the period to be described as the “age of fishes.” As in the case of the crustaceans, referred to above, we find one assemblage more or less peculiar to the freshwater or brackish conditions of the Old Red, and another characteristic of the marine Devonian; on the whole the former is the richer in variety, but there seems little doubt that quite a number of genera were capable of living in either environment, whatever may have been the real condition of the Old Red waters. Foremost in interest are the curious ostracoderms, a remarkable group of creatures possessing many of the characteristics of fishes, but more probably belonging to a distinct class of organisms, which appears to link the vertebrates with the arthropods. They had come into existence late in Silurian times; but it is in the Old Red strata that their remains are most fully preserved. They were abundant in the fresh or brackish waters of Scotland, England, Wales, Russia and Canada, and are represented by such forms as Pteraspis, Cephalaspis, Cyathaspis, Tremataspis, Bothriolepis and Pterichthys.

In the lower members of the Old Red series Dipterus, and in the upper members Phaneropleuron, represented the dipnoid lung-fishes; and it is of extreme interest to note that a few of these curious forms still survive in the African Protopterus, the Australian Ceratodus and the South American Lepidosiren,—all freshwater fishes. Distantly related to the lung-fishes were the singular arthrodirans, a group possessing the unusual faculty of moving the head in a vertical plane. These comprise the wide-ranging Coccosteus with Homosteus and Dinichthys, the largest fish of the period. The latter probably reached 20 ft. in length; it was armed with exceedingly powerful jaws provided with turtle-like beaks. Sharks were fairly prominent denizens of the sea; some were armed with cutting teeth, others with crushing dental plates; and although they were on the whole marine fishes, they were evidently able to live in fresher waters, like some of their modern representatives, for their remains, mostly teeth and large dermal spines, are found both in the Devonian and Old Red rocks. Mesacanthus, Diplacanthus, Climatius, Cheiracanthus are characteristic genera. The crossopterygians, ganoids with a scaly lobe in the centre of the fins, were represented by Holoptychius and Glyptopomus in the Upper Old Red, and by such genera as Diplopterus, Osteolepis, Gyroptychius in the lower division. The Polypterus of the Nile and Calamoichthys of South Africa are the modern exemplars of this group. Cheirolepis, found in the Old Red of Scotland and Canada, is the only Devonian representative of the actinopterygian fishes. The cyclostome fishes have, so far, been discovered only in Scotland, in the tiny Palaeospondylus. Amphibian remains have been found in the Devonian of Belgium; and footprints supposed to belong to a creature of the same class (Thinopus antiquus) have been described by Professor Marsh from the Chemung formation of Pennsylvania.

Plant Life.—In the lacustrine deposits of the Old Red Sandstone we find the earliest well-defined assemblage of terrestrial plants. In some regions so abundant are the vegetable remains that in places they form thin seams of veritable coal. These plants evidently flourished around the shores of the lakes and lagoons in which their remains were buried along with the other forms of life. Lycopods and ferns were the predominant types; and it is important to notice that both groups were already highly developed. The ferns include the genera Sphenopteris, Megalopteris, Archaeopteris, Neuropteris. Among the Lycopods are Lycopodites, Psilophyton, Lepidodendron. Modern horsetails are represented by Calamocladus, Asterocalamites, Annularia. Of great interest are the genera Cordaites, Araucarioxylon, &c., which were synthetic types, uniting in some degree the Coniferae and the Cycadofilicales. With the exception of obscure markings, aquatic plants are not so well represented as might have been expected; Parka, a common fossil, has been regarded as a water plant with a creeping stem and two kinds of sporangia in sessile sporocarps.

Physical Conditions, &c.—Perhaps the most striking fact that is brought out by a study of the Devonian rocks and their fossils is the gradual transgression of the sea over the land, which took place quietly in every quarter of the globe shortly after the beginning of the period. While in most places the Lower Devonian sediments succeed the Silurian formations in a perfectly conformable manner, the Middle and Upper divisions, on account of this encroachment of the sea, rest unconformably upon the older rocks, the Lower division being unrepresented. This is true over the greater part of South America, so far as our limited knowledge goes, in much of the western side of North America, in western Russia, in Thuringia and other parts of central Europe. Of the distribution of land and sea and the position of the coast lines in Devonian times we can state nothing with precision. The known deposits all point to shallow waters of epicontinental seas; no abyssal formations have been recognized. E. Kayser has pointed out the probability of a Eurasian sea province extending through Europe towards the east, across north and central Asia towards Manitoba in Canada, and an American sea province embracing the United States, South America and South Africa. At the same time there existed a great North Atlantic land area caused partly by the uplift of the Caledonian range just before the beginning of the period, which stretched across north Europe to eastern Canada; on the fringe of this land the Old Red Sandstone was formed.

In the European area C. Barrois has indicated the existence of three zones of deposition: (1) A northern, Old Red, region,