UNio. 281 



jS. Batavi. — Shell less thick than the preceding ; 

 the cardinal teeth oblique, less thick, some- 

 times conic ; hinder teeth generally thick, ele- 

 vated, but slightly elongate. 



7. Rostrati. — Hinge teeth generally thin and elon- 

 gate ; the hinder side of the shell ending in an 

 oblique more or less acute beak. 



I believe the first and last of these groups agree 

 with the two British species, and that the presumed 

 species are only varieties. 



