GEMMA. 419 



anterior in the left, and the posterior in the right valve 

 strong and somewhat bifid, the others simple and lamellar ; 

 lozenge prominent, obliquely sulcately crenate within. 

 Pallial line remote from the ventral margin, ending behind 

 in a short, narrow, triangular sinus. 



Ex. M. violacea, Schumacher, pi. 107, fig. 2, 2, a. 



There are but few species of Mercenaria described, all 

 from the shores of America. The valves of the shell 

 appear to be regarded with much interest by the Red men, 

 being used as ornaments for their dresses, and strung on 

 leathern thongs as treaty-belts or wampum. 



Species of Mercenaria. 



Mortoni, Conr. prseparca, De Kay. 



notata, Say. violacea, Schum. 



Genus GEMMA, Deshayes. 



Shell roundly triangular, sub -equilateral ; surface of 

 valves smooth, the margins crenulated. Hinge short, nar- 

 row, with three primary teeth in the left valve, the middle 

 one conical and slightly arched, and two in the right valve, 

 diverging, with a wide pit between them; ligament external. 

 Muscular impressions ventral ; pallial line marginal, with a 

 very long, narrow, deep sinus ascending perpendicularly. 



Ex. G. gemma, Totten, pi. 107, fig. 3, 3, a. 



This genus is founded on a pretty little shell from the 

 United States of America, in which there are only two pri- 

 mary teeth in the right valve, and where the sinus of the 

 pallial impression is deep and angular, as in Dosinia. 



