MESOZOIC TIME — TEIASSIC AND JURASSIC. 



781 



species, and Chemnitzia 20 per cent (Etheridge) . The genus Nerinea, having 

 one or more ridges in the spiral cavity (Fig. 1294) is confined to the Oolyte, 

 and the Cretaceous period. 



Cephalopods of the Ammonite type have an enormous expansion in the 

 period ; 250, or three-fifths of tlie British species, occur in the Lias. Figs. 

 1296, a are from the Lower Lias ; 1295 and 1297 from the Middle Lias ; 



1295-1297. 



129T 



Cephalopods (Ammonites) of the Lias. — Kg. 1295, Pleuroceras spinatum ; 1296, a, Coronioeras Bucklandi ; 1297, 



-lEgoceras capricoruus. 



1298, from the Inferior Oolyte ; 1299, from the Middle Oolyte. The last two 

 figures have the aperture unbroken ; and in 1299 it is much prolonged on 

 either side. 



1298-1299. 



1299 



Cephalopods of the Oolyte. — Fig. 1298, Stephanoceras Humphriesianum ; 1299, Cosmoceras Jason. 



Besides the Cephalopods with external chambered shells (Tetrabranchs), 

 the Belemnites (Dibranchs) (page 424) were of many species. Figs. 1302, 

 1303, represent the bones or osselets of two species, in their ordinary 

 broken state; and Figs. 1300, 1301, an unbroken one, in two different 

 positions. 



