50 INVERTEBRATA OF MASSACHUSETTS. 



GENUS THRACIA. 



Shell inequivalve, slightly gaping at both sides; beaks con- 

 spicuous, one of them perforated ; hinge with prominently thick- 

 ened margins, to which an external ligament is attached. 



THRACIA GONRA'PI. 



Shell tumid, thin, inequilateral, rounded-ovate, with the smaller 

 extremity truncated, exterior pale ashy-white, beneath which it is 

 pearly. 



State Coll., No. 219. Soc. Cab., No. 2318. 



Thracia declivis, CONRAD ; Amer. Mar. Conch., 44, pi. 9, f. 2. 



Thracia Conradi, COUTHOUY ; Bost. Journ. Nat. Hist., ii. 153, pi. 4, f. 2. 



Shell ovate, orbicular, rounded before, narrowed and trun- 

 cated behind, thin, light and fragile, of a dingy white color ; beaks 

 nearly central, protuberant, turned a little backwards, that of the 

 right valve perforated to receive the point of the other ; the 

 flattened area about the ligament is large and lance-shaped, bound- 

 ed by a distinct ridge ; surface coarsely wrinkled by the lines of 

 growth, undulated by a ridge extending from the beak to the lower 

 posterior angle, and by another broader eminence running from 

 the beak to the middle of the base, causing an outward curve to, 

 the margin at that part ; the right valve is considerably larger than 

 the left, projecting beyond it, and much more convex. Liga- 

 ment large, protuberant, and rounded, attached within to strong, 

 thick, rounded eminences,. which run backwards from the beaks 

 along the edge. Interior of a chalky white color ; palleal im- 

 pression broad, with a very deep, acute-angular sinus. A thin, 

 brownish epidermis is found on some portions of the shell. 

 There is no ossiculum. Length 3 inches, height 2J inches, 

 breadth 1| inch. 



Found perfect, and containing the living animal, on Chelsea 

 Beach, after violent easterly storms, a_nd probably lives in the sand 

 not far from low-water mark. Single valves have been found 

 abundantly on Rhode Island, and also on the coast of Maine, and 

 in Buzzard's Bay, so that it probably inhabits the whole New 

 England coast. 



This is a large and interesting shell, easily distinguished from most 



