In the greater part of them, the cup or pelvis, Figure 479, b, is greatly developed in size in proportion to the arms, although this is not the case in Fig. 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, and Platycrinus, are all of them characteristic of this formation. Other Echinoderms are rare, a few Sea-Urchins only being known: these have a complex structure, with many more plates on their surface than are seen in the modern genera of the same group. One genus, the Palæchinus (Fig. 480), is the analogue of the modern Echinus, but has four, five, or six rows of plates in the interambulacral region or area, whereas the modern genera have only two. The other, Archæocidaris, represents, in like manner, the Cidaris of the present seas.
Mollusca.—The British Carboniferous mollusca enumerated by Mr. Etheridge[[6]] comprise 653 species referable to 86 genera, occurring chiefly in the Mountain Limestone. Of this large number only 40 species are common to the underlying Devonian rocks, 9 of them being Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or Palliobranchiates). This latter group constitutes the larger part of the Carboniferous Mollusca, 157 species being known in Great Britain alone, and it will be found to increase in importance in the fauna of the primary rocks the lower we descend in the series. Perhaps the most characteristic shells of the formation are large species of Productus, such as P. giganteus, p. hemisphericus, P. semireticulatus (Fig. 481), and P. scabriculus. Large plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Fig. 482), also abound; and smooth species, such as Spirifera glabra (Fig. 483), with its numerous varieties.
Among the brachiopoda, Terebratula hastata (Fig. 484) deserves mention, not only for its wide range, but because it often retains the pattern of the original coloured stripes which ornamented the living shell. These coloured bands are also preserved in several lamellibranchiate bivalves, as in Aviculopecten (Fig. 485), in which dark stripes alternate with a light ground. In some also of the spiral univalves the pattern of the original painting is distinctly retained, as in Pleurotomaria (Fig. 486), which displays wavy blotches, resembling the colouring in many recent trochidæ.
Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but the majority, though often referred to as living types, such as Isocardia, Turritella, and Buccinum, belong really to forms which appear to have become extinct at the close of the Palæozoic epoch. Euomphalus is a characteristic univalve shell of this period. In the interior it is divided into chambers (Fig. 487, d), the septa or partitions not being perforated as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears to have retreated at different periods of its growth from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl. The animal of the recent Turritella communis partitions off in like manner as it advances in age a part of its spire, forming a shelly septum.
More than twenty species of the genus Bellerophon (see Fig. 488), a shell like the living Argonaut without chambers, occur in the Mountain Limestone. The genus is not met with in strata of later date. It is most generally regarded as belonging to the pelagic Nucleobranchiata and the family Atlantidæ, partly allied to the Glass-Shell, Carinaria; but by some few it is thought to be a simple form of Cephalopod.
