274 



HISTORICAL PALEONTOLOGY. 



forms of Ammotiitida constitute one of the most conspicuous 

 features in the life of the Chalk period. 



The Dibranchiate Cephalopods are represented partly by the 



Fig. 203. Turr:lites caie- Fig 204. a, Ptychoceras Einericianum, reduced 



natus. The lower figure rep- Lower Greensand ; b, Bacnlites anceps, reduced 



resents the entire shell ; the Chalk : c, Portion of the same, showing the folded 



upper figure represents the edges of the septa ; d, Crioceras cristatum, reduced 



base of the shell seen from Gault; e, Scaphitcs izqualis, natural size Chalk; 



below. Gault. f, Hamites rotundus, restored Gault. 



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

 by the internal skeletons of Belemnites. Amongst the latter, 

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



