362 EXPLORATIONS ACEOSS THE GREAT BASIN OF UTAH. 



buccal side truncated above from the beaks obliquely forward, rounding rather abruptly 

 into the base below; posterior side much narrower and longer than the other, and very 

 sharply rounded or slightly truncated at the extremity; base semiovate, being much 

 more prominent in the central and anterior regions than behind; dorsal outline declin- 

 ing from the beaks at an angle of about loo , the posterior slope being distinctly con- 

 cave. Beaks prominent, equal, incurved, and located half-way between the middle and 

 the anterior end; lunule deeply excavated, but not defined by a distinct marginal angle; 

 escutcheon lanceolate, rather deep, and circumscribed by a marginal ridge; surface 

 marked by fine lines of growth, with usually more or less distinct concentric ridges and 

 furrows. 



Length, 1.30 inches; height, 0.85 inch; convexity (of a right valve), 0.39 inch. 



This species is quite abundant, but, in all the specimens obtained, the hard calca- 

 reous matrix adheres so firmly about the hinge that it is impossible to clear it away so as 

 to see the teeth. Judging from the form of the shell, however, and the fact that it is 

 associated with fresh-water and estuary species, there is little room for doubt in regard 

 to its generic relations.* Most of the specimens are right valves;, a few left valves, 

 however, were obtained, which indicate that the species is only slightly inequivalve. 



This shell varies much in its surface-markings; some specimens showing only con- 

 centric stria 1 , and others concentric furrows and ridges. At first I thought there might 

 be two distinct species, separable on this character; but, after seeing large collections, I 

 found all intermediate gradations between these extremes, and united the two under 

 the first name. 



Locality ami position, same as last. 



Corbula Eng-elmanni, Meek. 



Plate 5, fig. 13, a, b. 

 Corbula (Potamomya) Engelmanni, Meek, (July, I860), Proceed. Acad. Nat. Sci. Philad., XII, 313. 



Shell rather small, transversely subovate, gibbous in the umbonal region; anterior 

 side narrowly rounded; base semiovate, being more prominent toward the front than 

 behind; posterior side narrow, and truncated at the immediate extremity, having a 

 moderately distinct angle extending from the back part of the beaks obliquely back- 

 ward to the lower part of the slightly truncated posterior end; beaks depressed, located in 

 advance of the middle; surface ornamented by small, very regular, concentric wrinkles; 

 hinge and interior unknown. 



Length (of a right valve), 0.39 inch; height, 0.21 inch; convexity, 0.11 inch. 



This little shell seems to differ materially in form from the last; but owing to its 

 small size, and the fact that specimens certainly belonging to that species vary in form, 

 I am not quite sure that it may not be a young example of the same. Until specimens 

 showing the intermediate connecting links can be found, however, I prefer to keep 

 them separate. 



Locality and position. — Same as last. 



*Long after writing th.- al.ov.-. I sm ,-. .-il.-d in working out the hinge, and found it to agree well with that of 

 Corbula, and not with Potaummm. Mr. I onra.l wrote me that he had proposed to found a g.-tm-, l«jW/„ /W r/,.#* for its 

 lvc-ption, mainly ou its apparent fresh-water hahits; hut I am not satisfied that it isgenerieally distinct from Corbula. 



