430 



MESOZOIC ERA— AGE OF REPTILES. 



V 



Fig. 652. 



Fig. 653. 



Fig. 650. 



Fig. 651. 



Figs. 648-653.— Jurassic Lamellibranchs and Brachiopods of England: 648. Ostrea Sowerbyi. 

 649. Pecten fibrosus. 650. Ostrea Marshii. 651. Khynchouella varians. 652. Terebratula sphae- 

 roidalis. 653. Terebratula digona (after Nicholson). 



Ammonites. — The Ammonite family, which is distinguished, as 

 already explained (p. 313,), by the dorsal position of the siphuncle 

 and the complexity of the suture, is represented in extreme abun- 

 dance by the type- 

 genus Ammonites. * 

 About 500 species 

 of this genus are 

 known, ranging in 

 time from the Trias- 

 sic through the Cre- 

 taceous. They are 

 therefore character- 

 istic of the Meso- 

 zoic. They varied 

 extremely in shape, 

 and in size from half 



Fig. 654.— Ammonites Humphreysianus. n , -■ 



an inch to a yard or 

 more in diameter. The accompanying figures represent some of the 

 most common species. 



In the genus Ammonites the distinguishing character of the family, 

 viz., the complexity of the suture, reached its highest point. In this 

 genus, the edge of the septa, which was only zigzag in Goniatite, and 

 lobed in the Ceratite, becomes most elaborately frilled. We give in 

 Fig. 659 the form of suture in the type-genera of the different orders 

 of shelled Cephalopods, in the order of their first appearance. In each 



* This genus is now broken up into many, but it is still convenient to retain the name 

 for a very distinct group of Cephalopods. 



