196 



AMERICAN JOURNAL 



This is the only land shell in the collection, and there can be 

 little doubt that it belongs to some section or subgenus of Buli- 

 mus, probably Plectostylus, Beck. The sculpture is so minute 

 that it requires a lens to make it visible. 



Cqncliifera. 



Family OORB ULWM. 



PACHYDON, Gabb. 



The hinge of this genus is ver}'- similar to that of Corbula, 

 much more so than to that of Azara^ but the spiral beaks are in 

 marked contrast to those of Corbula. When the left valve of 

 Corhula idonea, Conrad, is compared with a corresponding one 

 of Pachydon tenuis, they seem to be opposite valves, so widely 

 different is their outline. 



The pallial line is entire, not slightly sinuated, as Mr. Gabb 

 has defined it. The nympha is minutely rugose. 



The name Pachydon is objectionable, in consequence of its 

 derivation being the same as Pachyodon, and I have been re- 

 quested to substitute another. If naturalists object to Mr. Gabb's 

 name, I would suggest Anisothyris (unequal valves) to take its 

 place. Mr. Gabb referred this genus to the family Isocardiidce, 

 but, except in the turn of the beaks, there is nothing to suggest 

 such an arrangement. 



P. TENUIS, Gabb.— PI. 10, fig. 1. 



Subtriangular, very oblique, substance thick in adult speci- 

 mens ; right valve profoundly ventricose ; umbonal slope slightly 

 angulated, nearly terminal ; posterior extremity truncated ; car- 

 dinal tooth broad and thick, erect, curved, with an acutely angu- 

 lar margin ; this tooth is overlapped in front by a carinated and 

 sulcated projection ; lunular depression profound, very large and 

 broad. 



P. tenua, Gabb, Amer. Journ. of Conch, vol. 4, p. 199, pi. 16, 

 fig. 6. 



This shell has a very thin, light brown polished epidermis. 

 P. CARINATUS, Conrad. — PI. 10, fig. 7. 



Triangular, very inequivalved, right valve profoundly ventri- 

 cose, but flattened on the disk ; posterior slope flattened, having 

 an indistinct ridge in the middle, and forming nearly a right an- 

 gle with the umbonal slope, which is slightly carinated ; poste- 

 rior extremity acutely angular ; left valve prominently angular 

 on the umbonal slope, concave anterior to it, and depressed on the 



