232 ANATINID^ 



has more regularly proceeded than was the case with that 

 figured by Montagu, have been separated by Turton, and 

 established as distinct under the name truncata. Its 

 shape varies then from suborbicular, in which state the 

 lower margin is always more or less distorted or sinuated, 

 to subrhombiform, with the ventral edge perfectly free from 

 any irregularity. The valves are slightly unequal, and 

 somewhat ventricose, of a dull whitish hue externally, and 

 internally of a more or less pearly-white, which is some- 

 times even iridescent (varying in solidity from fragile to 

 moderately strong). The outer surface is minutely shagreen- 

 ed, appearing under a powerful glass crowdedly set with 

 microscopic grains of sand, and is more or less strongly 

 marked with concentric wrinkles of growth, which some- 

 times, though rarely, are elevated at irregular intervals into 

 fine laminae. The general inclination of the ventral edge 

 (which is variously indented) is to convexity, with a dis- 

 position to form a slight retusion towards the front. The 

 anterior dorsal margin is always peculiarly sloping, and 

 in the more perfectly and quietly developed specimens is 

 arcuated or at least convex ; the hinder dorsal edge is in- 

 ferior both in convexity and declination, being typically 

 scarcely sloping, and only slightly curved outward. Near 

 its extremity the sides are plainly unequal, the anterior one, 

 whose extremity is irregularly rounded or very obtusely 

 wedge-shaped, being invariably the shorter; and in the 

 rhomboidal shapes this inequality is so considerable, that 

 the hinder is often more than twice as long as the front 

 portion. The posterior termination is almost always more 

 or less obtusely angulated above, and frequently so beneath, 

 but is there as often rounded. The umbonal ridge is very 

 blunt and often obscure ; and the beaks are small, acute, 

 and incurved. The ligament is dejiressed, buft-coloured. 



