266 OSTREADiE. 



This very delicate and fragile Lima is of an uniform 

 pure and somewhat transparent white. Its valves, which 

 are alike hoth in size and profundity, are ohliquely sub- 

 ovate, and more or less swollen. The convexity, which 

 is chiefly manifest upon the umbonal region, diminishes 

 thence with tolerable evenness on either side; but from 

 the posterior lateral compression, the shell appears rather 

 deeper at that extremity. The united valves when exa- 

 mined sideways appear heart-shaped, owing to the distinct 

 but gradually diminishing ventricoseness of the valves 

 towards the base of the shell. 



The sculpture merely consists of fine radiating elevated 

 striae (with, for the most part, still finer Intervening 

 ones), which are simple (not distinctly granular or squa- 

 miferous), very numerous, chiefly perceptible and farthest 

 apart near the centre, more crowded and gradually disap- 

 pearing In front, and rather more abruptly obsolete pos- 

 teriorly. The posterior margin runs In a nearly straight 

 and almost uninterrupted line ; the anterior edge first 

 inclines forward in an arcuated line, then suddenly bend- 

 ing back, runs In a convex line to the somewhat at- 

 tenuated and well rounded ventral margin, so as to be 

 subparallel (although rather more oblique) to the opposite 

 edge. The chief anterior swell Is decidedly above the 

 middle, and most frequently, In the full-grown shell, 

 at about two-fifths the distance from the hinge-margin. 

 There is no posterior gape, and only a very slight linear 

 hiatlon at the front dorsal region. The hinge-margin Is 

 very short, not much more than a quarter of the width 

 of the shell, and declines with tolerable evenness on either 

 side. The auricles are tolerably distinct and well de- 

 fined ; both form obtuse angles, but the lateral outline of 

 the hinder Is more concave than that of the other. The 



