UNIO. 141 



liensive. Its valves are of an elongated ovate or ovate-ob- 

 long shape, thick, unequally ventricose, (the hinder portion 

 being always much more compressed than the frontal), and 

 with their umbones typically more or less swollen. The ex- 

 ternal surface, which has no other sculpture than the rugose 

 lines of increase (the umbonal wrinkles, which are subtuber- 

 cular and angularly disposed, alone excepted) is more or 

 less radiatingly painted all over, the colour of its epidermis 

 ranging from dusky olive to a clear ochraceous yellow 

 adorned with rays of a pure bluish green ; it is occa- 

 sionally, however, of an uniform olivaceous brown. The 

 ventral edge ascends in a well arcuated sweep anteriorly, 

 is either straightish or curved out a little in the middle, 

 and rising behind, often with some degree of suddenness, 

 in a slightly convex line, forms, with the upper margin, a 

 conical posterior beak, of which the bluntly-pointed apex 

 (occasionally a little truncated in the young) is either sub- 

 central or below the middle of the side. The dorsal edge, 

 typically rather short in proportion, is nearly straight and 

 level on both sides ; the upper posterior edge declines mode- 

 rately and is straightish, or slightly subretuse. The anterior 

 side, which occupies from one-quarter to one-third of the 

 entire length, is unsymmetrically rounded at its extremity, 

 the upper corner being often angulated from the comparative 

 straightness of the dorsal line, and the ventral ascent being 

 always more oblique than the upper declination. The liga- 

 ment is rather large and projecting ; the prominent umbones 

 are not usually broad, and have generally a well-marked 

 impression before them. The internal nacre seems almost 

 always of a bluish white (rarely of a salmon colour) ; the 

 primary teeth are strong, and a little compressed, the hinder 

 of the left valve being thick and conspicuously erect ; the 

 lateral laminae are produced and devoid of crenulatious. 



