68 



sities of the hinge large, arched, terminated near the 

 apices by a cavity sometimes very deep; ligament ex- 

 ternal, partly fixed beneath the apices. From the 

 shells of this genus possessing a lateral tooth, though 

 sometimes obsolete, and being covered with an epider- 

 mis, they are conjectured to inhabit rivers at their 

 junction with the sea. Shell equivalve, inequilateral, 

 of an oblique heart-shape. Apices obliquely curved, 

 three unequal primary teeth meeting at their bases, 

 and a little divergent upwards. 



Cyprina tenuistria Cyprina Islandica. 

 [And six fossil species.] 



CYTHERTA. 



venus casta. — Gmelin. 



Chemnitz 6, tab. 33, fig. 346. 

 This genus, notwithstanding the number of the spe- 

 cies, was blended with the G. Venus of Linnaeus, con- 

 tributing to render that an overgrown and badly de- 

 fined family. La Marck observes that a great diffi- 

 culty exists in discovering the characters of some of 

 the species, the shades of difference between them be- 

 ing so extremely slight; he however has pointed out, 



