472 



LAMELLIBRANCHIATA. 



In the sub-genus Gryplicea are included Oysters which were 

 either quite free or very slightly attached. The left or lower 

 valve (fig. 327) is much the largest, and has a very pro- 



ng. 326. Ostrea Marshii. Oxford 

 Clay (Middle Oolites). 



Fig. 327.Gryphcea incurva. Lias. 



nounced incurved beak, whilst the right valve is small and 

 concave. In the sub-genus JExogyra (fig. 328), again, the 

 beaks are "reversed" that is to say, turned towards the 

 posterior side of the shell. True Oysters commence to be 

 represented in the Carboniferous seas, abound in the Secon- 

 dary and Tertiary periods, and are very plentiful at the 

 present day. The sub-genera Gryphcea and Exogyra, are ex- 



rig. 32S.Exogym virgulu. Upper Oolites. 



clusively Mesozoic, the former abounding especially in the 

 lower portion of the Oolitic series, whilst the latter is chiefly 

 characteristic of the later Oolitic and Cretaceous deposits. 



Lastly, the genus Pernostrea has been founded for Juras- 

 sic Oysters, which differ from Ostrea proper in having the 

 ligament contained in from four to eight transverse grooves or 

 pits (as in Perna). 



In a second section or sub-family (Anomiadce) of the pres- 



