SCAPHARCA. 537 



many sub-equal plates ; the hinder tooth ovate, arched, 

 small, of a few irregular laniinse ; ligament external ; liga- 

 mental area narrow, smooth. Pallia! impression entire. 



Ex. N. reversa, Gray, pi. 125, fig. 1, I, a. 



In general appearance the shells of Noetia are like 

 those of reversed Anadane. The species are few, and are 

 from the coasts of America. 



Species of Noetia. 



contraria, Reeve. reversa, Gray. 



ponderosa, Say. 



Genus SENILIA, Gray. 



Shell solid, sub-cordate, equilateral, equivalve ; surface 

 of valves strongly radiately ribbed, and covered with a 

 thin, olive, polished epidermis. Hinge with the teeth large, 

 oblong, nearly similar, divided into large rugose, irregular 

 cross plates ; ligament external ; ligamental area rather 

 broad, with concentric cartilage grooves. Pallial im- 

 pression entire. 



Ex. S. senilis, Linnceus, pi. 125, fig. 2, 2, a. 



The only species of Senilia known is met with in the 

 brackish waters of Africa. 



Genus SCAPHARCA, Gray. 



Shell ovate or oblong, sub-quadrangular, sub-equilateral, 

 inequivalve, closed in front ; surface of valves strongly cos- 

 tate, and covered with a paleaceous epidermis. Hinge with 

 the teeth sub-equal, dilated, and with the plate of the outer 



