JURASSIC ANIMALS. 



431 



case the suture is supposed to be divided on the ventral surface and 

 spread out, so that the central part in the figure represents the dorsal 



Fig. 657. 



Fig. 658. 



Figs. 055-658.— Jurassic Cephalopods— Ammonites: 655. Ammonites bifrons. 656. Ammonites 

 margaritanns. 657. Ammonites Jason: a, side view; b, showing suture. 658. Ammonites cor- 

 datiis: a, side view; b, showing suture. 



portion, and the two extremities the ventral. The evolution of form 

 and structure was in the following order : First the straight OrtJwceras, 



Ammonite. 



yfUl 



So 



m 









-&> 





Ceratite. 



NautUoids, 



Orthoceratite. 



Fig. 659.— Diagram showing Form of the Suture. Position of Siphuncle, and Order of First Appear- 

 ance of Families in Cephalopods. 



