222 ANATINID^. 



but slightly convex line, is sometimes, but not invariably, 

 a little retuse, adjacent to the obtusely-biangulated pos- 

 terior extremity of the shell. The anterior side is rounded; 

 the posterior narrowed, and always decidedly the shorter, 

 although the range of variation as to the extent of in- 

 equality is tolerably extensive. The front dorsal edge is 

 typically arcuated, but in some specimens is decidedly 

 sloping, and in others is but slightly convex. The hinder 

 dorsal line slopes but little, and after being excavated for 

 the insertion of the ligament, curves out gently to the sub- 

 truncated and linearly gaping posterior extremity. The 

 beaks are inflected and oblique, the umbone of the larger 

 valve prominent, and the ligament short, somewhat sunken, 

 and of a yellowish olive colour. The imibonal ridge is very 

 distinctly defined ; a shallow excavation divides it from the 

 dorsal edge. The hinge consists of a simple, small, not very 

 thick triangular plate, which is slightly hollowed out upon 

 the top, excepting at its broader end, which causes that 

 portion to appear j^ossessed of a denticle, or incipient tooth. 

 This plate is not divided by any medial tooth, and, being 

 entirely posterior, leaves the umbonal cavity exposed. The 

 ossicle, or moveable shelly plate, (which lies immediately 

 beneath the beaks in front of the hinge-plate, clasping the 

 edge of either valve so as to present to the eye, when at- 

 tached, only its arcuated edge,) is moderate in size, and of 

 a broad crescent-shape. When detached (and, unfortu- 

 nately, this almost always occurs before its capture, or else 

 in the excision of the animal,) it appears to be very convex 

 on one side, but flat, with a large and deep excavation near 

 the concave edge, on the other. It never attains to any 

 considerable size, and rarely exceeds the dimensions of the 

 specimen we have delineated. 



"Animal oval, elongated, moderately thick, having the 



