182 CORBULID^. 



lustre, and the smaller one is clothed with an umber-brown, 

 tolerably thick epidermis, beneath which the surface is 

 whitish, or a very pale pink madder colour ; its only sculp- 

 ture consists of a few rather distant elevated lines, which 

 radiate, with but little divergence, down the central area of 

 it. The larger valve is of a squalid white, not unfrequently 

 adorned, in fine examples, with more or less broad rays of 

 darker or paler crimson, and closely grooved throughout 

 with simple concentric sulci. The ventral margin is more 

 or less arched, and is typically straighter, and more ascend- 

 ing posteriorly, but is very variable in outline, although 

 usually more tumid in front, from whence proceeds that 

 slight obliquity so generally apparent in its contour. The 

 sides are rarely equal, but usually the anterior, though 

 occasionally the posterior, is the more produced. The dor- 

 sal edges are nearly straight, and moderately, but decidedly 

 sloping, the amount of declination not being widely different 

 on either side. The front extremity is rounded, and the 

 hinder one attenuated and obtusely biangulated, the poste- 

 rior edge is slightly convex. The umbonal ridge is obsolete ; 

 the beak of the lesser valve is acute : and there is a strong 

 though undefined depression on either side of the umbones. 

 The interior is almost always devoid of colour ; and the 

 palleal scar, without forming a distinct sinus, makes at the 

 posterior extremity a very slightly obtuse angle with the 

 former line of its direction. In the right valve is a posterior, 

 strong, simple, and somewhat recurved, pointed primary 

 tooth, with an adjacent cavity in front, which is partly 

 occupied by the cartilage : in the left valve, behind the 

 receptacle for the opposite tooth, is a kind of excavated 

 one, of which the middle portion is hollowed out for con- 

 taining the cartilage ; but the basal and especially the 

 hinder rim is elevated above the dorsal surfoce. 



