LAMELLIBRANCHIATA. 



221 



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

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

 and Exogyra are exclusively Mesozoic, the former abounding 



Fig. 171. Ostrea Marshii, Oxford 

 Clay (Middle Oolites). 



Fig. 172. {jfyjmcea tucurva, 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 Aiiomia the shell is thin and translucent, and is fixed 

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



Fig. 173. Pgcten 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 



