220 ANATINID^. 



THRACIA, Leach. 



Shell transversely ovate, tumid, or compressed, generally 

 thin, inequivalve, often nearly equilateral, more or less 

 gaping posteriorly ; surface nearly smooth, or minutely 

 scabrous, sometimes covered by an epidermis. Muscular 

 scars unequal, united by a strongly sinuated palleal im- 

 pression. Hinge composed of a thickened shelly, more or 

 less prominent fulcrum in each valve ; ligament partly in- 

 ternal ; cartilage furnished with a free crescentic ossicle. 

 Beaks entire. 



Animal ovate. Mantle closed, except for the passage of 

 a compressed linguiform foot. Siphons rather long, sepa- 

 rated to their origins, and furnished with fimbriated orifices, 

 which are "often inflated into a globular form" (Clark). 

 A single branchial lamina developed on each side. Labial 

 appendages triangular, pectinated. 



The shells of this genus are not remarkable for beauty, 

 being all white or dusky in colour, and not distinguished 

 by any conspicuous ornaments. The scabrous, or shagreen- 

 like structure, which the surfaces of many of them present, 

 has been investigated by Dr. Carpenter, who finds it to 

 depend upon " the presence of numerous large isolated 

 cells, filled with calcareous matter, which form a superficial 

 coating superposed upon the ordinary external layer, the 

 periostracum being continued over them, and sinking down 

 into their interspaces, just as the human epidermis covers 

 the papillary surface of the true skin. The proper exter- 

 nal layer is composed of polygonal cells, with sharply 

 defined boundaries having large nuclear spots." 



