Page:The Geologist, volume 5.djvu/37

Rh The genera found in the two counties were not all confined to the Devonian period. The following table shows their Chronological distribution so far as the Silurian, Devonian, and Carboniferous deposits of Britain are concerned.

From which it appears that twenty-four genera,—about one-fourth of the whole series,—are peculiar to Devonian deposits, fourteen common and restricted to the Silurian and Devonian, forty-one common to all three, and eighteen common and restricted to the Devonian and Carboniferous; hence a total of fifty-five Devonian genera occur in the preceding, and fifty-nine in the succeeding period. Some of the genera occur in Neozoic deposits, and a few in the existing Fauna.

When the numbers of species contained in each of the forty-one genera of the fourth column (Table VI.) are tabulated in parallel columns for the three periods, the figures present themselves in four different principal forms of succession, as may be illustrated by taking the genera Favosites, Cyathophyllum, Loxonema, and Orthoceras.

Sil. Dev. Carb. Favosites 8 5 la descending-despending series. Cyathoi)hylluin 9 14 8 ascending-descending. Loxoneina 2 8 14 ascending-ascending. Orlhoceras 55 12 35 descending-a:5cending.

The first kind shows that the maximum specific development occurs in the Silurian era, the second in the Devonian, and the third in the Carboniferous; the fourth kind may perhaps be regarded as a sort