SOLEN. 247 



though usually of moderate thickness, and are scarcely, if 

 at all, curved. The exterior is almost smooth, or at most 

 concentrically striolate, and is covered with a membrana- 

 ceous, strong, highly polished, yellowish-green, or oliva- 

 ceous yellow epidermis, beneath which the shell is whitish, 

 with irregular interrupted concentric bands of a purplish 

 liver colour. The colouring is most profusely displayed 

 upon the hinder area, which is diagonally divided from the 

 other by an imaginary line drawn from the beaks to the 

 posterior corner, behind which the surface is slightly more 

 elevated, again becoming suddenly flattened towards the 

 dorsal margin, where the edges have a tendency to bend 

 outward, instead of curving inwards. The dorsal and 

 ventral margins are almost parallel, both sloping a little 

 upwards at the rather narrower anterior extremity. This 

 latter, as well as the posterior termination, is truncated, 

 the edges descending abruptly, yet not without some de- 

 gree of convexity, especially in front, to the ventral margin. 

 There is no stricture nor internal callus in front, neither is 

 there any attenuation of the valves at the hinder extre- 

 mity, which, on the contrary, is broad, and almost rectan- 

 gularly biangulated. The inside is white, exhibiting in the 

 thinner and younger specimens the external colouring. 

 The hinge, which is terminal, and whose entire length is 

 not equal to one half of the ligamental callus, is furnished 

 in the left valve with two rather solid and closely approx- 

 imating fang-shaped primary teeth, which admit between 

 them the peculiarly thin and laterally compressed tooth of 

 the other. This latter, when in fine preservation, (which 

 is rarely the case, being almost invariably broken in ex- 

 tracting the animal) is bifid at its apex, and strengthened 

 at its base in the more delicate individuals by a more or 

 less distinct and semicircular horizontal plate. There are, 



