AMMONITID^E. 



289 



semble Goniatites in having a discoidal shell, the coils of which 

 lie in one plane and are contiguous. It is distinguished, how- 

 ever, from Goniatites on the one hand and Ammonites on the 

 other, by having the 

 " lobes " of the suture 

 denticulated or cren- 

 ulated, whilst the 

 " saddles " are simply 

 rounded. The species 

 of Ceratites are typi- 

 cally Triassic, the best- 

 known form being the 

 C. nodosus of the 

 Muschelkalk. Some 

 species, however, oc- 

 cur in the Cretaceous 

 Rocks, though no 

 member of the genus 

 has as yet been de- 

 tected in the intervening Jurassic deposits. 



The genus Ammonites comprises by far the greater number 

 of the Ammojiitidcz, over five hundred species being already 

 known. The shell in Ammonites is spirally rolled up into a 

 flat spiral, all the volutions of which are contiguous (figs. 



Fig. 259. Ceratites nodosus. 

 (Middle Trias). 



Muschelkalk 



Fig. 260. Ammonites Humphresianus. Inferior Oolite. 



260, 261). The innermost whorls of the shell are more or less 

 concealed ; the septa are undulated ; the sutures are lobed, 

 foliaceous, or ramified; and the siphuncle is dorsal. The 

 species of the genus Ammonites range from the Trias to the 



