144 UNIONIDJE. 



tinental forms of this polymorphal shell, any of which our 

 Islands do not exhibit a nearly analogous representative. 

 If a certain platitude of language be demanded in a descrip- 

 tion of the preceding species, far more highly requisite is it 

 that our diagnosis of the present one should be sufficiently 

 inclusive. 



The contour varies from a produced oval to elongated 

 oval oblong, the texture from actually solid (it is rarely, if 

 ever, so in our British examples) to rather thin and fragile. 

 The valves are ventricose, but the profundity is rather dif- 

 fused, the umbonal region in place of tumidity more fre- 

 quently exhibiting a sort of compression, which, carried 

 downwards to the basal margin, usually displays itself in a 

 greater or lesser retusion of that portion of the shell. No 

 sculpture, unless the rugose lines of increase be regarded as 

 such, adorns the exterior ; but two radiating series of small 

 isolated (not confluent) narrow tubercles are visible upon 

 the umbones. The epidermis is of an olivaceous yellow 

 (for the most part changing into green posteriorly, near the 

 obscure umbonal fold), is generally zoned with brown at the 

 stages of growth, and rarely, if ever, displays any distinct 

 radiation at any other portion of its surface. The dorsal 

 and ventral margins run almost parallel ; the former is 

 almost horizontal, and nearly upon the same level on either 

 side of the beaks ; the latter rises obliquely and arcuatedly 

 in front, but ascends very gently, if at all, at the posterior 

 end. Similarly the declination of the posterior margin in 

 the more typical examples is very moderate, and the edge 

 itself short in proportion to the hinder dorsal outline ; 

 hence the termination of the shell is never cuneiform as in 

 tumidus, but the tapering portion is generally short, and 

 the beak, whose tip is either below the middle or at most 

 subcentral, is more or less obtusely subtruncated. The 



