THRACIA. 69 



for a broad, compressed foot, extending tlic whole length of tlie 

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

 wrinkled ; siphons long, slender, separate in their whole extent. 



Tliis genns, proposed by Mr. Couthony, has, I observe, been ad- 

 mitted l)y J. E. Gray, in the "Annals of Science," and I have 

 therefore adopted it without hesitation. 



This species very closely resembles Mya (Cochlodesma) prcr- 

 fcriuis of Pennant (Ligida prcEtenuis, Montagu), but differs in Iteing 

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

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



Oeiiits THRACIA, Leach. 1824. 



Shell inequivalve, slightly gaping at l)oth ends ; beaks consinc- 

 uous, one of them perforated ; hinge with prominently thickened 

 margins, to Avliich an external ligament is attached. 



Thracia Conradi. 



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

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



Thracia declivis, Conkad, Aincr. Marine Coiu'Ii. 44, pi. 9, fig. 2. 



Thracia Conradi, Coutiiouv, Bost. Journ. Nat. Hist. ii. 153, pi. 4, fig. 2. — De Kay, 

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

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

 England, 23; Im. Gr. Manan, 21. — Reeve, Eleui. Conch ii. 154, fig. 130. 



Shell ovate, orbicular, rounded before, narrowed and truncated 

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

 central, protuljerant, 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 |)0ste- 

 rior angle, and l)y another broader eminence running from the 

 beak to the middle of the l)ase, causing an outward curve to the 

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

 the left, projecting l)eyond it, and much more convex. Ligament 

 large, protuV)erant, and rounded, attached within to strong, thick, 

 rounded eminences, which run backwards from tlie beaks along 

 the edge. Interior of a chalky white color; ]>allial impression 

 broad, with a very deej), acute-angular sinus. A thin, 1)rownish 



