274 



HISTORICAL PAL/EON TOLOGY. 



forms of Ammonitida constitute one of the most conspicuous 

 features in the life of the Chalk period. 



The Dibranchiate Cephalopods are represented partly by the 



Fig. vn.T*rr!Ktes cate- 

 natus. the lower figure rep- 

 resents the entire shell ; the 

 upper figure represents the 

 base of the shell seen from 

 below. Gault. 



Fig. 204. a, Ptychoceras Emericianum, reduced 

 Lower Greensand ; b, Bacnlitcs anceps, reduced 

 Chalk : c, Portion of the same, showing the folded 

 edges of the septa ; d, Crioceras cristatnm, reduced 

 Gault; e, Scaphitcs aqnalis, natural size Chalk; 

 f, Hamites ro tun Jus, restored Gault. 



beak-like jaws of unknown species of Cuttle-fishes and partly 

 by the internal skeletons of Belemnites. Amongst the latter, 

 the genus Belemnites itself holds its place in the lower part of 



