Page:A Treatise on Geology, volume 1.djvu/99

 CH. V.

The unequal periods of existence of different genera are here very apparent. Producta, after abounding in Devonian and carboniferous ages, perishes in the saliferous period. Spirifera passes through all these periods and ends in the oolitic; but terebratula occurs through all the strata, and still lives.

Most of the fossil cephalopoda belong to extinct genera: of these, bellerophon and orthoceras are abundant in the lower and middle palaeozoic strata. Hamites, scaphites (almost peculiar to the cretaceous system, a few only in the oolites), and Belemnites, belong to the mesozoic series, and characterise the oolitic and chalk rocks exclusively.