540 arcing. 



Genus scaphula, Benson. 



Shell thin, elongate, sub-trapeziform, equivalve, very 

 inequilateral, hinder slope keeled, closed in front ; surface 

 of valves covered with a thin, smooth epidermis. Hinge 

 with the teeth narrow, enlarged at the outer end, and 

 divided into a few broad, oblique, nearly longitudinal 

 laminse ; ligament external ; ligamental area narrow. 

 Pallial impression entire. 



Ex. S. pinna, Benson, pi. 125, fig. 6, 6, a. 



The two species of Scaphula known are found in the 

 rivers of India, one in the Ganges and its branches, a 

 thousand miles from the sea, and the other in the Tenas- 

 serim, Birmah. 



Species of Scaphula. 

 celox, Bens. pinna, Bens. 



Genus ARGINA, Gray. 



Shell sub-globose, sub-cordate, sub -equivalve, inequi- 

 lateral ; surface of valves strongly radiately costate, and 

 covered with a brown, foliaceous epidermis. Hinge with 

 the hinder tooth elongated, curved, the central crest small ; 

 ligament external ; ligamental area narrow, smooth. Pal- 

 lial impression entire. 



Ex. A. pexata, Say, pi. 125, fig. 7, 7, a. 



The character by which the shells of Aryina may be 

 easily known is the curved and shortened anterior tooth, 

 the plates of which are crowded together and irregular. 

 The species are inhabitants of the coasts of America. 



