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GENUS THRACIA, LEACH. 



THE THRACIA. 



Generic characters. Shell ovate, oblong, transverse, ine- 

 quivalve ; the beaks strongly recurved, that of the right valve 

 notched to receive that of the opposite side ; the right valve 

 more convex and larger than the left ; the hinge has upon 

 each valve a horizontal and narrow nympheal callosity, which 

 sometimes expands into a spoon-shaped projection, and con- 

 tains an internal ligament, which is prolonged, and slightly issues 

 outwardly into the corselet. Two muscular impressions, the 

 anterior depressed, elongated, contracted towards its middle, 

 and united to the posterior, which is rounded, by a palleal 

 impression, strongly notched at its posterior part. 



Testa ovato-oblonga, transversalis, inaequivalvis ; natibus fortiter recur- 

 vatis, dextrae valvae emarginata nate, quse alterium excipiat ; dextra valva 

 ventricosior majorque sinistra : Cardo variabilis ; in utraque valva callum 

 nymphale, horizontale, angustum, aliquando dilatatum cochleariforme ; 

 Ligamentum internum, externe prominulum. Duae impressiones rauscu- 

 lares; anterior depressa, producta, contracta ad medium, junctaque poste- 

 riori ; posterior rotundata impressione palleali, postice largiter emarginata. 



The animal is oval, thick, enveloped in a very delicate 

 mantle, diaphanous upon its sides, through which is distinctly 

 seen the branchiae and the abdominal mass; the edges are 

 swollen, lobed, and united in almost the whole of the circum- 

 ference, so that they present only three openings ; the first is 

 found at the anterior and lower third of the median line, and 

 Thracia. A 



