1 50 GASTROCILENIDiE. 



valves are moderately convex, but compressed subcentrally. 

 Externally the colour is dirty-whitish, internally of a very 

 slightly nacreous porcelain white. The outer surface is nearly 

 smooth, but minutely and irregularly wrinkled concentri- 

 cally at the narrower extremity, and in some specimens ap- 

 pears delicately and distantly sublaminated, through a kind 

 of imbricated elevation of the former stages of growth. This 

 latter feature can, however, be only regarded as an accidental 

 and not a permanent characteristic. The ventral margin is 

 nearly straight, with sometimes a slight retusion towards 

 the front ; the anterior dorsal edge slopes but very mode- 

 rately, and almost in a straight line ; the hinder dorsal line is 

 elevated above the level of the beaks, ascending from them 

 with a very trifling slope, in nearly a rectilinear or scarcely 

 convex course, and uniting near its close with the posterior 

 margin in a bold and arcuated outward sweep to the lower 

 corner of the hinder extremity. The hinder side is thus 

 dilated, and contains nearly quadruple the area of that of 

 the anterior : this latter, which is not more than half as 

 long as the posterior, is narrow, attenuated, and obtusely 

 rounded at its commencement. The umbonal ridge is re- 

 markably developed, running very obliquely and most 

 prominently, so as to cut off a large posterior convex space. 

 Although much elevated, it is not acute, and does not exhibit 

 the slightest trace of any aculeation. Before it, a somewhat 

 triangular surface, occupying about the same space as that 

 succeeding it, is depressed below the level of the remainder 

 of the shell. The beaks are distinct, but small, and appa- 

 rently do not incline to either side ; the umbones appear 

 depressed when examined from the posterior side ; but are 

 much raised above the level of the front dorsal margin. 

 There is no vestige of a lunule, and the ligament is almost 

 minute, of an ashy hue, and scarcely projecting. No teeth 



