78 AMERICAN JOURNAL 



concave area, transversely striated. Exterior of the valve with 

 two obscure carinae extending from the beaks to the anterior 

 corners of the shell, and another, even less evident, in the me- 

 dian line. The posterior portion of the shell creamy white, ex- 

 treme edges lightly horny ; area between the two carina of a 

 fine brown, stronger on the lines of growth and diminishing pos- 

 teriorly ; median carina paler. Surface polished, with obscure 

 impressed lines and slight rugosities under a lens. 



Interior whitish, not polished. When fresh all the organs are 

 represented in a faint impression on the inner surface, but this 

 becomes obscure when the shell is contracted by drying. In- 

 ternal laminae prominent. In all the specimens of this genus 

 which I have seen the right lamina in the neural valve is a little 

 longer than the other. 



The muscles are attached on the inner concave edges of the 

 laminae. The muscular impressions are very small and obscure.* 

 Length of shell 1*7 inches, greatest width 0*55 inches. Length 

 of peduncle 4*5 inches. Habitat at the head of the Gulf of 

 California on the Lower Californian side, opposite the mouth of 

 the Colorado River ; in sandy shelly mud at low water mark. 

 Ten specimens, with the animal in alcohol, were collected by 

 Dr. Edward Palmer. Cabinets Smithsonian Institution, W. H. 

 Dall, Philadelphia Acad. Nat. Sciences, Thomas Davidson, Esq. 

 and the Boston Society of Nat. History. 



This magnificent species was collected alive by the indefatiga- 

 ble exertions of Dr. Palmer, who spent many hours groping in 

 the mud after the small number of specimens above recorded. 



I should have no hesitation in considering it a distinct species, 

 coming as it does from a distinct zoological province, from the 

 Gr. albida, but that the number of specimens of the latter (very 

 rare species) which I have seen, is quite limited. 



The principal differences are as follows : 



The albida, as before described, agrees with the descriptions of 

 Hinds, Reeve and Sowerby. The shell under my eyes differs 

 from them in being much more elongated and narrow, propor- 

 tionally, than the albida ; the beaks are mere acute, the internal 

 laminae are closer together and less widely divergent. The pe- 

 duncle is very much longer, the brown color so conspicuous on 

 the exterior is not found in Gr. albida, which also has a much 

 more prominent median carina and a more solid shell which does 

 not gape. 



On the whole, the shell in question, if not a distinct species, as 

 I am inclined to believe, is at least a very strongly marked va- 



*The scars of the post-adductor are thickened and plate-like. 



