MARINE MOLLUSCA OF THE UNITED STATES. 179 



here, because Dr. Stimpson has included it in his check-list of 

 recent species, yet I suspect that it has not been found living. 



5. A. pexata, Say. Fig. 466. 



Journ. Philad. Acad., ii. 2G8. 1822. 

 Shell covered (when fresh) with a hairy epidermis, transversely 

 subovate, with from 32 to 36 ribs, placed closer together than their 

 own diameters ; beaks far forward, near the anterior termination 

 of the hinge approximate. 



Length 57, height 43 mill. 



Rhode Island, southwards. 



6. A. Americana, Gray. Fig. 470. 



Wood, Index Test. Suppl., t. 2, f. 1. 

 Shell ovately oblong, sides rounded, the anterior verj T short 

 and contracted ; white, covered with a rather thick blackish-brown 

 epidermis, which is bristty in the interstices between the ribs ; ribs 

 about 35 in number, each one with a median impressed line, inter- 

 stices deeply cut ; ligament area very narrow ; umbones anterior, 

 nearly touching. 



North Carolina to West Indies. 



This shell is allied to A. pexata,, but is somewhat larger, more 

 oblong in shape, and the ribs are generally impressed. Mr. Reeve 

 says (Conch. Icon.) that the ribs of Americana are flat, while 

 those of pexata are impressed in the middle, but the contrary is 

 the case in nearty all the specimens I have examined. 



7. A. Holmesii, Kurtz. Fig. 471. 



Cat. Mar. Shells, 5. 1860. 



Distinguished from the two preceding species by its smaller 

 size, and more inflated and globular form ; it is also more solid. 

 Inhabits estuaries (Kurtz). 



N. and S. Carolina. 



8. A. incongrua, Say. Fig. 472. 



Journ. Philad. Acad., ii. 268, July, 1822. 



Shell somewhat squarely orbicular, rather thin, very inequi- 

 valve, sides angulated at the upper part, anterior side the longest ; 

 white, with a thin light brown epidermis ; ribs 27 or 28 in number, 

 in the left valve the anterior ones are elevately crenated, in the 

 right valve they are all crenated; ribs of the left value rather 

 broader than those of the right ; ligament area rather wide ; um- 

 bones somewhat approximating. 



Length 2-f , height 2 inches. 



N. Carolina to W. Indies. 



