220 



MOLLUSCA. 



Lobes of the mantle entirely separated ; the foot small and 

 byssiferous, or wanting. A single adductor muscle. 



In the typical Oysters, forming the genus Ostrea (figs. 169, 



Couloni, Lower Greensand. ' 



170), the shell is irregular, and is attached by the left valve, 

 which is also convex, and has a well-marked beak. The upper 

 valve is generally flat or concave, and is the smallest of the 



Fig. 170. Ostrea aquila, Lower Greensand. 



two valves. The hinge is toothless. Both valves may be 

 more or less completely plain, and the upper one especially 

 often is so. The lower valve, however, is commonly plaited, 

 and both valves are sometimes thus ornamented, as in Ostrea 

 Marshii of the Oolites (fig. 171). 



In the sub-genus Gryphaa are included Oysters which were 

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

 valve (fig. 172) is much the largest, and has a very pronounced 

 incurved beak, whilst the right valve is small and concave. In 

 the sub-genus Exogyra, 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 



