224 ARCADE. 



form. Its valves arc much compressed, thin, brittle, ami 

 almost semitransparent, smooth, with the exception of the 

 presence of a few growth lines, and covered with a highly 

 lustrous epidermis, which varies in hue from olivaceous 

 yellow to yellowish drab or ash colour, but is not rayed or 

 variegated in any individuals we have met with. The 

 sides are, as usual, extremely unequal, and the ventral 

 edge, which is not cremated internally, greatly arcuated. 

 The anterior edge is more or less straight and abrupt ; the 

 hinder edge at first rather ascends than declines, so that a 

 more or less subangulated curve prevents a continuity and 

 abruptness in the slope ; hence the posterior termination is 

 not subcuneiform, or much attenuated, but simply, though 

 not broadly, rounded. The greatest breadth of the shell is 

 a little before the middle, and not immediately behind the 

 umbones. The hinder dorsal area is not flattened, so that 

 the lips project rather than incurve ; there is no sculpture 

 either there or on the site of the lunule, which latter is but 

 seldom at all defined. The umbones are not prominent ; 

 the beaks are small, but acute and distinct. The internal 

 nacre is silvery white, and in general not remarkably bril- 

 liant ; the edge is entire ; there are about six teeth before 

 the cartilage-pit (which is extremely oblique), and fifteen 

 behind it. The Greenland specimens are described by 

 Moller as having twelve anterior and sixteen posterior 

 teeth ! (Ind. Moll. Groenl., p. 17.) One of our larger 

 examples measured four-tenths of an inch in length, and 

 one-fourth less in breadth. The younger individuals are 

 more elongated in proportion. 



The animal is white : the margins of its mantle are quite 

 free in front and posteriorly, and simple at edge. The 

 foot is white, rather more elongate, and not so markedly 

 pedunculated as in the other British Nucul<e, nor are the 



