192 AMERICAN JOURNAL 



DESCRIPTIONS OP NEW WEST COAST SHELLS 



BY T. A. CONRAD. 



MACTRID^. 



HARVELLA, Gray, 



H. PACTFICA, Conrad. 



Description. — Equilateral, ventricose ; posterior extremity 

 situated midway between the summit and ventral margin, which 

 is regularly and profoundly rounded ; post umbonal area narrow 

 and depressed. 



Inhabits Panama. 



Observations. — Differs from the Florida species, H. elegans, in 

 being less ventricose, having much finer and closer ribs on the 

 umbo, in having the posterior angle situated much higher, in 

 having a narrower post-umbonal area ; in the anterior cardinal 

 cavity being much wider, and in the ribs being less distant. 



I have no doubt of the independent origin of these two species, 

 nor do I believe that the Isthmus faunoe of the two oceans con- 

 tained originally, if they do now, any identical species. C. B'. 

 Adams thought there was one shell common to both oceans — 

 Crepidula unguiformis — but as that shell is occasionally rayed 

 in the Panama specimens, and never in the Atlantic species, I 

 have no doubt of their being distinct. It has been supposed 

 that the two oceans communicated in the Miocene Period, but, 

 in that case, there would have been little distinction between the 

 shells of each ocean ; but, in fact, the Miocene analogues are of 

 the Pacific type on the west, and of the Atlantic type on the 

 east, where they resemble recent American shells. Since the 

 Miocene period, there is no evidence of a subsidence, and I be- 

 lieve that it was the elevation of land at the close of the Oligocene 

 Period that separated the Atlantic and Pacific oceans. I find 



