364 MACTRID.E. 



rupt one, of the posterior side. The ventral margin is 

 more or less regularly subarcuated ; the sides are equal, 

 and not very unsymmetrical ; they taper a little at their 

 extremities, which are not regularly rounded, both (espe- 

 cially the hinder one) displaying a more or less slight angu- 

 lation near the lower margin. The umbones are rather 

 prominent ; the beaks are more or less acute, lean for- 

 ward, and do not approach closely to each other. The 

 umbonal fold is sufficiently evident, but neither carinated 

 nor peculiarly angulated. The interior varies from pale lilac 

 to white ; the teeth are very large, but thin, the cartilage 

 pit is rather narrow, and the lateral laminae, which are of 

 about the same size and distance, are perfectly free from 

 crenatlon. 



The dimensions of rather a large example were, an inch 

 and five eighths in breadth, and rather more than two 

 inches in length. The dorsal lips of the young specimens 

 project greatly, and their arcuation is very considerable. 



The following full account of the animal was drawn up 

 by Mr. Clark this summer, and kindly communicated to 

 us by that gentleman, on the value of all whose observa- 

 tions we lay the greatest stress. "Animal suboval, tumid, 

 of the palest bluish-white, with its mantle entirely open, 

 the margins thereof clothed with an intensely white deli- 

 cate fringe, formed of linear short filaments ; the ventral 

 portion of the body is white, and the dorsal, containing 

 the mass of the liver and other viscera, of a duller and 

 more opaque white ; the siplional tubes are short, of the 

 same length, and united to their extremities, and are 

 never, in their utmost extension, exserted more than three- 

 fifths or half an inch ; the orifices of the tubes are truncate 

 and furnished with pale, dirty- red cirrhi, the branchial 

 rather the longest in diameter, with from twelve to six- 



