THE LAMELLIBRANCHIA 69 



waves or streams, and retain a recognizable form while 

 most of their associated fossils are pulverized. Hence 

 they are often found as derived fossils in strata much 

 younger than those to which they belong ; Jurassic oysters, 

 for instance, are very commonly found in the glacial 

 gravels of the Pleistocene period, 



9. Corbula is a genus represented by various small 

 species in the Jurassic, Eocene and Oligocene strata of 

 Britain, such as C. pisum and C. revoluta of the Barton 

 Clay. The shell is inequivaive, the right valve being in 

 every way larger than the left though the two scarcely 

 differ in form ; both being oval and rostrate posteriorly, 

 with no ornament but lines of growth. The umbones are 

 nearly central in position and opisthogyral. The hinge- 

 structure is best made out on large species such as Corbula 

 gallica of the Eocene of the Paris Basin (Fig. 21). In the 

 right valve there is a sharp anterior-cardinal tooth pro- 

 jecting to an unusual extent towards the left valve and 

 slightly upwards. Behind this is a space with a resilium- 

 pit which instead of being vertical is horizontal, facing 

 downwards, and placed within the umbo; behind this 

 there is a long vestigial posterior-lateral tooth. In the 

 left valve, a deep conical socket corresponds to the right 

 cardinal tooth ; behind this is a conspicuously-projecting 

 horizontal plate (resiliophore), which carried the resilium 

 and underlies the resilium-pit of the right valve. The 

 interiors are not nacreous; the shell is isomyarian and 

 has a short pallial sinus with vertical front edge. 



Modern species of Corbula are marine, living in 

 moderately shallow water, but an allied genus Erodona 



