LAMELLIBRANCHIATA. 



221 



seas, abound in the Secondary and Tertiary periods, and are 

 very plentiful at the present day. The sub-genera Gryphaa 

 and Exogyra are exclusively Mesozoic, the former abounding 



Fig. I7i. Ostrea Marshii, Oxford 

 Clay (Middle Oolites). 



ryptuea incurva, Lias. 



especially in the lower portion of the Oolitic series, whilst the 

 latter is chiefly characteristic of the later Oolitic and Cretace- 

 ous deposits. 



In the AnomicR the shell is thin and translucent, and is fixed 

 to some solid body by a plug which passes through a hole or 



Fig. 173. Pecten Islandicus, left valve. Post-Tertiary and Recent 



notch in the right valve. The typical fossil species are dis- 

 tributed from the Oolites upwards. The Oolitic genus Placu- 

 nopsis is also related to Anomia. 



The genus Pecten includes the Scallops (fig. 173), in which 



