Page:The Geologist, volume 5.djvu/29

Rh 'Catalogue,' the Heteropodous Mollusca are, in Table I., included in the class Gasteropoda.

It is scarcely necessary to remark that the fossils of Devon and Cornwall do not fully represent the organisms of the Devonian age, as seven other classes—Pisces, Pteropoda, Cirrepedia, and Annelida, amongst animals, and Cellulares, Monocotyledones, and Polycotyledones amongst plants—have been found in rocks of this age elsewhere; and of these the two first and the fifth have been met with in other British localities. The reptiles Steganolepis and Telerpeton, of the Elgin Sandstone, are not enumerated here, as some doubt attaches to the question of their chronology, if indeed they are not certainly Triassic. The single articulated class, Crustacea, is by no means rich in any way; with one exception, all its genera are Trilobites, and commonly contain but one species each. The most important class numerically is Brachiopoda, to which one hundred and eight species belong, that is, thirty-one per cent, of the entire series. The families and genera of Cephalopoda are richer in species than those of any other class, averaging sixteen for each family, and ten for each genus.

The most striking fact in this connection is the specific abundance of Brachiopoda and Cephalopoda, and the paucity of the classes Lamellibranchiata and Gasteropoda, as compared with the numerical rank of the same classes in the existing Fauna. This fact will, perhaps, be most strikingly exhibited by the following table, which has been thus computed: in the left-hand column the aggregate number of the species of fossil mollusca found in Devon and Cornwall has been put = one thousand, and the numbers belonging to each class computed to this; the right-hand column has been formed on the same principle, and is based on the data given by Forbes and Hanley in their 'History of British Mollusca.'

It appears, then, that within existing British seas the Lamellibranchiates are about twenty-four times more numerous specifically, than the Brachiopods, whilst within, what may be called, the same area, the latter were to the former, during the Devonian period,