GEXEEA OF AMMOXITID.E. 



77 



ceous rocks, tliough no member of the genus has as yet been 

 detected in tiie intervening Jurassic deposits. 

 The comprehensive 



\, 





y 



./■ 



genus Ammonites 

 comprises by far the 

 greater number of the 

 Ammonitidm, several 

 hundred species being 

 already known. The 

 shell in Ammonites is 

 spirally rolled up into 

 a flat spii^al, all the 

 volutions of wMch are 

 contiguous (figs. 464- 

 469). The innermost 

 whorls of the shell 

 are more or less concealed ; the 

 the sutures are lobed, foliaceous, or ramified ; and the 

 siphuncle is dorsal The species of the genus Ammon- 

 ites range from the Trias to the Chalk, and are thus 

 exclusively confined to the Secondary period. (It should 



Fig. 463.— Lt 



•atites nodoms. Muschelkalk 

 (iliddle Trias). 



septa are undulated 



Fig. iQi.— Ammonites Hmnphresianvs. Inferior Oolite. 



be remembered, however, as before said, that species of 

 Ammonites are found in Xorth America in beds which 

 some believe to belong to the Eocene Tertiary.) Within 

 these limits, each rock-grotip is characterised by partic- 



