CARDIUM. 27 



subsquamular, more or less compressed, close-set, sloping 

 down posteriorwards, and not extending transversely over 

 the entire ribs. The interstitial lines, separating the ribs 

 from each other, are extremely delicate near the middle of 

 the valves, and even at their extremities are still very 

 narrow ; these lateral ones are traversed by slightly- 

 elevated crowded concentric wrinkles, which are some- 

 times slightly indicated at the base of even the central 

 stria). The ventral margin is much arcuated, greatly 

 ascending at its posterior termination (and thus narrowing 

 the shell at that extremity), and not straightened in front. 

 The anterior dorsal edge declines but moderately, and is a 

 little convex ; the posterior dorsal edge is very short, very 

 slightly convex, and slopes downwards in a very trifling 

 degree. The posterior outline is barely convex, and the 

 extremity, though a little rhomboidal, is rounded both 

 above and below ; the front extremity is likewise rounded, 

 but is a little angular above. The beaks are situated much 

 nearer to the posterior end, to which they usually lean, and 

 are acute and moderately projecting. There is no trace of 

 a lunule, and the ligament is very small, narrow, and only 

 slightly prominent. The interior partakes of the external 

 colouring ; the lateral teeth are of moderate size, and sub- 

 equidistant ; the front one is rather the more approximate. 

 It is intermediate between C. nodosum and 0. exiguum, 

 but is a far thinner shell than either of them. Its outline 

 is much more oblique than the former, and its central ribs 

 are devoid of the graceful sculpture which adorns the 

 entire surface of that species. It more closely approaches 

 the latter, but its convexity is more evenly diffused, its 

 umbonal ridge not sharply angulated, and its ribs more nu- 

 merous and less elevated. These, in cxiguum, are se- 

 parated from each other by far wider grooves, which are all 



