88 KELLIADiR. 



the length, which latter outline is necessarily accompanied 

 by a greater declination of the dorsal edges. 



The ordinary shape, then, is subrhombic, with the angles 

 softened down ; the shell is ventricose or inflated (if much 

 produced, it is comparatively compressed), very thin and 

 fragile, moderately inequilateral, and of a transparent white, 

 under a very delicate glossy yellowish epidermis, which, 

 in certain specimens, faintly reflects prismatic colours. 

 The surface is almost smooth, and not distinguished by any 

 other sculpture than more or less developed concentric 

 striula; ; it is, moreover, not polished, but faintly shining. 

 The ventral and hinder dorsal edges are more or less sub- 

 parallel, although inclining a little towards each other ; 

 the former is convex at the extremities (where it ascends 

 nearly equally on either side), but a little straightened in 

 the middle ; the latter barely convex and (except in the 

 abbreviated variety) scarcely sloping. The hinder side, 

 which occupies about two-thirds of the shell, is broad and 

 very bluntly subbiangulated at its extremity, the posterior 

 edge not being arcuated, but, at most, convex. The front 

 extremity, which is rather the narrower one, is almost 

 symmetrically rounded in the more produced forms, and 

 bluntly rounded in those which are less elongated ; the 

 lower corner is invariably rounded, the upper less habitu- 

 ally so ; the front dorsal edge is straightish near the um- 

 bones, then more or less convex, and, except in the abbre- 

 viated form, declines but very moderately. The umbones 

 are rather prominent and incline slightly forward ; the 

 beaks are small, very acute and hardly lean to either 

 side. There is no lunule, nor any inflection of the dorsal 

 surface. The interior is white, with a large elongated 

 somewhat triangular rather oblique brownish-yellow liga- 

 ment, situated close under the beaks on the posterior side, 



