Page:The Geologist, volume 5.djvu/36

22 The fifty-six genera thus limited are generally poor in species, the aggregate number belonging to them being no more than ninety-two; that is, fifty-eight per cent, of all the genera contain no more than twenty-six per cent, of the total number of species. Forty- one of these fifty-six genera contain each but a single species in the British Devonian deposits. The only genera thus limited that can be said to be rich in species are Stromatopora, Acervularia, and Clymenia. The first, a genus of Amorphozoons, contains five species, all limited to South Devon; the second, a group of corals belonging to the great Palæozoic family Cyathophyllidæ, contains five species, all peculiar to South Devon; and the last, a genus of Cephalopod mollusks belonging to the family Nautilidæ, contains eleven species, all found at South Petherwin, not one being met with elsewhere in Britain. With the single exception of Cyrtoceras rusticum, found at South Petherwin—and this probably a synonym for Orthoceras arcuatum—the genus Cyrtoceras is restricted to South Devon, where it is represented by twelve species.

The distribution of the ninety-seven genera of fossils found in the two counties is exhibited in the following table:—

Every genus of the classes Amorphozoa, Zoophyta, and Brachiopoda occurs in South Devon, and with the exception of Cephalopoda it contains a greater number of genera in each class than either of the other areas. All the genera of Cephalopoda appear at South Petherwin.