lines crossing the transverse wi'inkles : ligament very short ; 

 antetior tip angulated : jJO'Sterior margin rounded, desti- 

 tute of distinct kinule : epidermis pale yellowish-olivace- 

 ous, radiated with green, interrupted into short lines and 

 spots, which are sometimes sagittate, and disappear on the 

 margins of the old shell. 



OBSERVATIONS. 



It is highly probable that the present species is the cunea- 

 iiis of Swainson, it certainly agrees with his descrip- 

 tion. But this is of little consequence, as Mr. Barnes' 

 name has the priority, and he has also preoccupied the 

 name which Mr. Swainson has chosen. 



Mr. Barnes correctly remarks that this shell, in shape 

 resembles Al,as:moijonta marginata, Nob. The extraor- 

 dinary depressions of the anterior margin is very charac- 

 teristic and readily distinguishes it from other species. As 

 the shell increases in age this margin becomes a little ele- 

 vated at tip, as is exhibited in our figures, and the position 

 of the beaks varies a little. The umbonial slope is often 

 elevated into a rib, and the striai occupy a considerable 

 part of the shell and become so profound as almost to den- 

 ticulate the edge of the shell. 



REFERENCE TO THE PLATE. 



Interior and exterior views of the riglit valve — the two upper figurce. 

 Anterior truncated margin — lower figure. 

 PLVrE IV . c 



