Ammonoidea — Ammonites. 



35 



of the shell is rarely obtained (Fig. 62). Samites attains its QAJiXEY 

 greatest development in the Gault. In Hamulina and Ptychoceras 

 (Neocomian and Gault) there is but one sharp bend in the shell, go, Vall^ 

 the straight limbs in Ptychocerm being actually in contact at, and caw 3, 

 in the region of, the aperture. 



Fig. 61. — Macroscaphites Ivanii, D'Orb. Lower Cretaceous. 



A still further departure from the typical form of the Cephalopod 



shell is encountered in the singular genus Turrilites, which takes 



the form of a Gasteropod shell (Fig. 63). The mouth or aperture 



is, however, turned towards the left-hand side, or, in other 



words, the shell is ** sinistral." Turrilites is found exclusively 



in Cretaceous rocks. Helicoceras is coiled like Turrilites, but the Wall-ease 



3 Tablfr- 

 whorls are disconnected. In Heteroceras the last whorl is detached g^g^ gg 



Fig. 62.— ITamites 



, D'Orb. Gault: Folkestone. 



(Fig. 64). Both are Cretaceous genera. In Baculites (Cretaceous) Table- 

 the shell is perfectly straight, except in the earliest or embryonic °*"® 

 stage of its development, in which it is coiled. It occurs in vast 

 numbers in the Danian (Upper Continental Chalk) of the North 

 of France, whence the name Baculite Limestone has been given 

 to those beds (Fig. 65). 



The next group, Ptychitid^, consists for the most part of Wall-oase 

 Triassic genera, but its earliest representatives come from the cage 89 



