The Bryozoa become much more important than they had been before, and contribute materially to the formation of the limestones. A characteristic Carboniferous genus is the screw-shaped Archimedes (XI, 7), while Fenestella continues to be very abundant.
The Brachiopoda have undergone a marked diminution, as compared with those of the Devonian, though they are still very common. Genera of long standing, like A try pa and Pentamerus, have died out, but others, like Chonetes (XII, 5), Spirifer (XI, 8; XII, 7), and Rhynchonella, are still represented, but most important of all the Carboniferous genera is Productus (XI, 11; XII, 6), which has a very large number of species, among them P. giganteus, the largest known brachiopod. Syringothyris (XI, 10) and Reticularia (XI, 9) are allies of Spirifer; while Meekella (XII, 4) and Derbya (XII, 3) are extreme developments of the Strophomenoid stock, of Ordovician origin. The genus Tere-bratula, which became exceedingly abundant in the Mesozoic periods, has its beginning in the Carboniferous genus Dielasma (XII, 9), though we have already found the family represented in the Devonian and Silurian.
The Bivalves are somewhat more abundant than in the earlier periods. Examples of these are Aviculopecten (XII, 13), Monopteria (XII, 11), and Myalina (XII, 12). Of Gastropods, the same genera that occur in the Silurian and Devonian are continued into the Carboniferous, such as Bellerophon (XII, 17), Euomphalus, Pleurotomaria (XII, 18), Loxonema, Platy-ceras, with the interesting addition of the most ancient land-shells yet discovered. The genus Conularia, referred to the Pteropods, is common. Among the Nautiloid Cephalopods, Ortho-ceras still persists, but this group reaches its acme in the number and variety of the coiled shells, many of which represent new genera, such as Cycloceras, Trigonoceras, etc. These Nautiloids have shells ornamented with prominent ridges or tubercles. The Ammonoids continue to be represented by Goniatites, but the Carboniferous forms of this group, such as Brancoceras (XI, 12) and Prodromites (XI, 13), display an advance over those of the Devonian in the greater complexity of their sutures, looking forward to the remarkable condition attained in Mesozoic times.
It is in this group that the most marked advances of Carboniferous life are to be observed, and the incipient stages of Mesozoic development are clearly shown. The extraordinary and bizarre Ostracoderms have become extinct, though the Arthro-dirans continue into the coal measures.
The Selachians are numerous and varied, having developed so enormously that they give the Carboniferous fish-fauna a very different aspect from that of the Devonian. Acanthodes is a small shark covered with a dense armour of exceedingly minute square scales, and the fins are supported by heavy spines along their anterior borders. Another remarkable shark is Pleuracanthus (a Permian species is shown in Fig. 279), which has many features in common with the Dipnoi, such as the shape of the tail, the character of the pectoral fins, and the bones which form the roof of the skull, while the skin is naked. Isolated fin-spines and teeth show that many other kinds of sharks existed in the Carboniferous, in some of which the teeth were converted into a crushing pavement, adapted for a diet of shell-fish. (See PI. XI, Fig. 14).
The Dipnoi continue, though in diminished numbers, and their most prominent representative is the genus Ctenodus.
The Crossopterygians are much less abundant than in the Devonian; the commonest American genus is Ccelacanthus, which, though unmistakably a member of this group, has assumed the form of a bony fish, and looks much like a chub.
The Actinopteri are still represented only by the Ganoid cohort; these hold their own and even increase their numbers, many new genera replacing those of Devonian times. Eurylepis, Palceo-niscus, Eurynotus, and Cheirodus are the best-known genera; they are all of moderate size and in appearance are not strikingly different from modern fishes.
The Amphibians, which we have seen some reason to believe existed in the Devonian, are of greatly increased importance in the Carboniferous. At the present time the Amphibia are represented by the dwarfed and specialized frogs and toads, newts and salamanders, which give but an imperfect notion of the structure of the extinct members of the class. The Carboniferous Amphibia all belong to the extinct order Stegocephalia, in which the skull is well covered with a roof of sculptured bones, and which are of moderate or small size, not exceeding seven or eight feet in length and mostly much smaller. The backbone is not ossified, the limbs are weak, the tail short and broad, and in many forms the belly is protected by an armour of bony scutes. An extraordinary number of genera of Carboniferous Stegocephalia are known, most of them like the Salamanders in shape, but some are elongate, slender, and of snake-like form. Examples are Archegosaurus,Branchiosaurus, Dendrerpeton, Ptyo-niusj and many others.