OYRENID^. 185 



tooth in each valve ; pallial sinus shallow. Perhaps scarcely 

 subgenerically distinct from Cyrena. For a doubtfully creta- 

 ceous species. D. tenuis (cxv, 32). 



ISODOMA, Desh. Shell thin, fragile, resembling a dementia, 

 elongately ovate, moderately tumid ; hinge of right valve with 

 two bifid diverging cardinal teeth, and one distinct remote 

 lateral tooth on either side ; pallial sinus slight. Based on I. 

 cyrenoides^ Desh. (cxv, 33, 34) ; Paris Basin. It recalls Clementia 

 in general character, but the dentition of the hinge is distinctly 

 that of Cyrenidffi. 



Yelorita, Gray, 1834. 



Distr. — 3 sp. Philippines, India, Japan. V. Gijprinoides, 

 Gray (cxiv, 39, 40}. 



Shell with epidermis, cordiform, triangular, thick, solid ; three 

 cardinal teeth, the anterior one of the right valve and posterior 

 of the left rather small ; lateral teeth large, very finely striated, 

 the anterior very large, triangular, the posterior compressed, 

 elongated. 



Batissa, Gray, 1841. 



Distr. — 30 sp. Polynesia, Australia, E. Indies. B. violacea, 

 Lam, (cxiv, 41-43). 



Shell subcordiform, solid, covered with a horny, greenish 

 epidermis ; three cardinal teeth in each valve, the right anterior 

 and left posterior ones rather small ; lateral teeth compressed, 

 striated, the anterior very short, posterior elongated. 



CORBICULA, Muhlf., 1811. 



Distr. — 120 sp. India, East Indies, Philippines, So. America. 

 G. cor. Lam. (cxiv, 44-46^. Fossil. Laramie; N. America. 



Shell subcordiform, solid, close, concentrically striated or 

 ridged, covered b}^ a smooth, greenish epidermis; three cardinal 

 teeth in each valve, the right anterior and left posterior rather 

 small; lateral teeth elongated, compressed, striated; ligament 

 prominent, thick; pallial impression with a slight or well-marked 

 sinus. 



VELORITINA, Meek, Shell thick, gibbous, obliquely cordate- 

 trigonal; beaks elevated, obliquely incurved, and tumid ; poste- 

 rior umbonal slopes very prominently rounded ; posterior dorsal 

 margins strongly incurved ; cardinal teeth typical, excepting 

 in being more oblique ; lateral teeth with striae very minute Or 

 obsolete, the posterior one of the left valve often appearing as 

 if merely formed by the beveled edge of the incui'ved dorsal 

 margin ; ligament small and depressed far below the elevated 

 umbonal slopes ; surface concentrically striated, Gorbicula 

 Durkeei, Meek, 



LEPTESTHES, Meek. Shell transversely elongate-subovate, com- 

 pressed, typically extremely thin, very oblique ; beaks depressed, 

 13 



