THRACIA. 69 



for a broad, compressed foot, extending the whole length of the 

 small, abdominal mass ; edges of the mantle a little thickened and 

 Avi-inkled ; siphons long, slender, separate in their whole extent. 



This genns, proposed by Mr. Conthony, has, I observe, been ad- 

 mitted by J. E. Gray, in the " Annals of Science," and I have 

 therefore adopted it without hesitation. 



This species very closely resembles Mya (Cocldodesma) pr(B- 

 tenuis of Pennant (^Ligula prcetenuis, Montagu), but differs in being- 

 more rounded, less convex, less narrowed behind, and has no 

 signs of a granulated or shagreened epidermis, like that shell. 



Genus THRACIA, Leach. 1824. 



Shell inequivalve, slightly gaping at both ends ; beaks conspic- 

 uous, one of them perforated ; hinge with prominently thickened 

 margins, to which an external ligament is attached. 



Thracia Conradi. 



Shell tumid, thin, inequipartitc, rounded-ovate, with the smaller extremity 

 truncated, exterior pale ashy-white, beneath which it is pearly. 



Thracia dcclivis, Conrad, Amer. Marine Conch. 44, pi. 9, fig. 2. 



Thracia Conradi, Couthouy, Best. Journ. Nat. Hist. ii. 1.53, pi. 4, fie:. 2. — De Kay, 

 Nat. Hist. New York, 237, pi. 28, fig. 284. — Eussell, Essex Journ. Nat. Hist, 

 i. 75. — MiGHELS, Bost. Journ. Nat. Hist. iv. 315. — .Stimpson, Shells of New 

 England, 23; Inv. Gr. Manan, 21. — Reeve, Elem. Conch ii. 154, fig. 130. 



Shell ovate, orliicular, rounded before, narrowed and truncated 

 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, bounded by a 

 distinct ridge ; surface coarsely wrinkled by the lines of growth, 

 undulated by a ridge extending from the beak to the lower poste- 

 rior 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. Ligament 

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

 rounded eminences, Avhich run backwards from the beaks along 

 the edge. Interior of a chalky white color ; })allial impression 

 broad, \nt\\ a very deep, acute-angular sinus. A thin, In'ownish 



