264 OSTREAD^. 



and subcliaphanous, white in the young, but rather stronger 

 and more opaque in the larger single valves which are 

 occasionally discovered. When viewed sideways, its 

 united valves appear distinctly heart-shaped ; the swell is 

 chiefly manifest upon the middle portion of the shell below 

 the umbones, and diminishes rapidly and nearly equally 

 on either side of them. The valves which are closed on 

 both sides, are divided in the middle by a concave ra- 

 diating grooA^e, on either side of which the surface is 

 set with very fine and closely crenulated radiating cos- 

 tellse, which do not extend to the extremities of the 

 shell, but for the most part occupy only the middle 

 portion of it. The interstices, which are larger than the 

 costellse, are not squarely cut out, but slightly shelving, 

 which causes the latter to become more or less acute 

 at their upper edges. The shape of the shell is nearly 

 elliptical, and its sides are of nearly similar curvature, 

 both arching almost uninterruptedly to the attenuated 

 and well-rounded ventral margin ; the greatest length of 

 the valves is scarcely above the middle. The hinge- 

 margin is nearly equal to one-third of the breadth, and 

 scarcely declines in the least on either side of the pro- 

 minent umbones, whose apices (the beaks) are rather 

 distant, and directly inflected, not leaning to either side. 

 The ligameutal area is moderately wide, its central oval 

 (in fresh specimens) is orange-yellow, the remaining por- 

 tion is light green. The auricles are of equal size, and 

 distinctly angulated, the angles being rather more than 

 rectangles. 



The largest single valve in our cabinet measures slightly 

 beyond seven lines in width, and rather exceeds one-third 

 of an inch in length ; although occasionally still larger 

 valves are met with, the average of living examples can 



