 



Ammonoidea — Ammonites^, 



37 



The family of the Artetid^ embraces a large number of Am- GAILEEY 

 monites which at first sight appear to be only remotely related ; . " 



nevertheless, a careful study of their development has led to their g 9 n . 



Tabie- ' 

 cages 62-9. 



Fio. 66 — Cardioceras cordatum, Sby. Oxford Clay. 



being grouped together. Among the most familiar members of 

 this large group are the following, viz. : Psiloceras planorbis, which 

 gives its name to the " Eone of Ammonites planorbis^^ of the 

 liower Lias, and at the same time marks the first occurrence of 

 Ammonites in British rocks; Arietites BucTclandi {A. Bucklandi 

 zone) ; Arietites ohtusus, Fig. 67 {A. ohtusus zone) ; and Oxynoticeras 

 oxynolum [A. oxynotus zone). 



Fig. 67. — Arietites obtusus, Sby. Lower Lias. 



The shells in this group are flattened in form, and the whorls 

 usually only slightly embracing, and generally numerous (see 

 Fsiloceras planorhis, e.g.). Oxynoticeras oxynotum is remarkable 



