236 ANATINID^E, 



Liffula prcsienuis, Mont. Test. Brit. Siippl. — Brown, 111. Conch. G. B. p. 106\ 

 pi. 42, f. 1. — Recll'z, Revue Zool. Soc. Cuvi'crr. 1045. 

 p. 416. 



CocModesma prcetemie, Couthouy, Boston Journal Nat. Hist. 1839. 



Thracia prcetcniiis, Loven, Index Moll. Scandin, p. 47. 



This scarce and interesting bivalve, wliicli appears to be 

 little known to the naturalists of the continent, is of a 

 somewhat oval or of an elongated ovate form, and rather 

 tortuous ; it is thin and brittle, rather pellucid, compressed, 

 and of an uniform scarcely glossy subnacreous white. The 

 surface, which when recent is covered by an extremely 

 delicate pale yellowish epidermis, is almost smooth, or only 

 faintly striolate, (being merely marked with the lines of in- 

 crease,) except behind the obsolete umbonal ridge, where it 

 is rendered minutely scabrous by almost microscopic pa- 

 pillae. The lower edge is convex, or even subarcuated, 

 in front, but rises behind where it is very faintly retuse. 

 The sides are nearly equal ; the anterior, which is rather 

 the longer, tapers but little, and is regularly rounded at its 

 extremity ; the posterior termination, which in the lesser 

 valve is rounded and attenuated, is subtruncated in the 

 larger one. The front dorsal margin is very much arched, 

 and slopes but very moderately ; the hinder dorsal edge, 

 which declines rather more, especially in the left valve, is 

 straightish, or occasionally even subretuse, at its commence- 

 ment, but a little convex at its extremity. The umbones 

 which are moderately prominent, are much inclined ; the 

 beaks are small and inflected. There is an obtuse iim- 

 bonal ridge, at whose origin near the beaks is a short linear 

 fissure, and internally a very narrow rib-like fold, the tri- 

 angular space near which is brilliantly pearly, a character 

 more peculiarly marked in the dead specimens, from the 

 general dulness of the surrounding surface. Both extre- 

 mities are a little gaping. The hinge consists of an hori- 



