THE LAMELLIBRANCHIA 61 



crowded out altogether. The posterior-laterals, again, 

 are a late development, resulting from the shortening-up 

 of the opisthodetic ligament : in early forms with a long 

 ligament they are wanting. 



Cyrena is a freshwater lamellibranch, ranging from 

 Jurassic to Recent. Modern species live only in sub- 

 tropical and tropical streams and mangrove-swamps, but 

 the genus (including the sub-genus Corbicnla, in which the 

 lateral teeth are cross- striated) survived in Britain until 

 well on in Pleistocene time. 



5. Crassatella sulcata is a common fossil in the 

 Barton Clay (Upper Eocene) of the Hampshire Basin. 

 In shape it is much like Trigonia, but the umbones 

 are forwardly directed (prosogyval). The surface is 

 ornamented with strong concentric ridges, but many 

 other species of the genus are nearly smooth (e.g., the 

 large C. plumbea of the Paris Basin, Fig. 17). There 

 is an escutcheon, and in front of the umbo there 

 is a very similar, but smaller, depressed semicircular 

 area, the lunule. Internally the shell is not nacreous ; it 

 is isomyarian and integripalliate ; the valve-margins are 

 crenulate (though not in all species of the genus). The 

 hinge-plate is very narrow in front and behind, but very 

 deep in the umbonal region, where it bears a ligament- 

 pit, much larger in proportion than that of Nucula. In 

 front of this are two nearly-vertical cardinal teeth : 

 those of the left valve (20, zb) are both large, those of the ; 

 right valve fit in front of those of the left, and the anterior 

 tooth (3#) is very small. The articulating surfaces of 

 the teeth are slightly cross-grooved like those of Trigonia^ 



