ASIPHONIDA. 



471 



SECTION A. ASIPHONIDA. 



FAM. 1. OSTREHLE. Shell inequivalve, slightly inequi- 

 lateral, free or attached ; hinge usually edentulous. Ligament 

 internal. 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. 324, 



Fig. 324. Ostrea Couloni. Lower Greensand. 



325), 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 two valves. The hinge is toothless, with a single 



Fig. 325. Ostrea aquila. Lower Greensand. 



ligamental pit. 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. 326). 



