170 PALEONTOLOGY. 



specimens I am indebted to Professor Henry. Dr. White has also identi- 

 fied it among Lieutenant Wheeler's collections from the north fork of Virgin 

 River, Utah. 



CORBULID^. 



Genus CORBULA, Brug. 

 CoRBULA (Anisorhynchus) pyeiformis, Meek. 



Plate 17, figs. 2, ami 2 a, b, c, rf. 



Corbula {Potamomyaf) pyriformis, Meek (1860), Proceed. Acad. Nat. Sci. Philad., 312. 



Corbula {Potamomya ?) concentrica, Meek (1860), ib., 313. 



Corbula {Anisorhynchtts) pyriformis, Meek (1871), Extract from Dr. Hayden's Second 

 Anu. Report GeoL Survey of the Territories; and (1872) in the last-mentioned 

 Reiwrt, 292; and (1876) in Col. Simpson's Report Expl. across the Great 

 Basin of Utah, 359. 



Corbula pyriformis, Dall (1872), Am. Jour. Conch., VII, 90. 



Shell attaining a large size, rather thick, oval-subpyriform, nearly 

 eqirivalve, very gibbous in the central and anterior regions, and much more 

 compressed, narrowed, and produced posteriorly; beaks elevated, nearly 

 equal, incurved, and placed more or less in advance of the middle; ante- 

 rior side generally truncated obliquely forward above, from the beaks to 

 near the middle, thence rounding abruptly to the base; posterior side much 

 attenuated, and usually slightly truncated at the immediate extremity; 

 dorsal margin generally very concave in outline behind the beaks, and pro- 

 vided with a well-defined marginal carina, extending in each valve from 

 the beaks nearly to the posterior extremity, and between these carinse with a 

 deeply-excavated lanceolate escutcheon; basal margin deeply rounded in 

 the central and anterior region, and more or less sinuous in outline behind 

 the middle; luntde deeply and rather largely impressed, without being 

 always distinctly defined, though it is sometimes margined by a subangular 

 ridge on each side. Surface ornamented with concentric ridges and fiuTows, 

 most regularly and strongly defined on the umbonal region, and gradually 

 becoming more irregular and less distinct toward the basal margin, or in 

 some cases entirely fading away, so as to leave only the lines of growth 

 over the whole exterior. Hinge with the tooth of the right valve rather thick, 

 prominent, subtrigonal, striated, and a little curved upward; cartilage-pit 

 fleep and trigoii;il; hinge of left valve, with pit and cartilage-process, pre- 

 senting the usual characters; pallial line with ai)parently a small shallow 



