366 NORTH AMERICAN INDEX FOSSILS. 



XLVII. Cypris Miiller. 



Reniform or oval, thin, translucent shells, with somewhat thick- 

 ened hinge margins ; ventral surface often sinuate ; surface smooth, 

 punctate or hirsute. Jurassic-Recent. (Fresh water.) 



ii6. C. purbeckensis E. Forbes. Jurassic-Cretacic. 



Subreniform, arched dorsally, nearly straight, or somewhat in- 

 curved ventrally, broadly and obliquely rounded in front; edge 

 view acute-oval; surface smooth, contact margins simple. 



Morrison of Colorado; Bear River of Wyoming; common in the 

 , English Purbeck beds. 



XLVIII. Cytherella Jones. 

 Shell oblong or subovate, compressed, especially in front ; valves 

 unequal, thick, generally with an even, smooth surface, but occa- 

 sionally undulating and ornamented with pits or granules ; contact 

 margin of the larger valve grooved on its inner edge for the recep- 

 tion of the flange-like edge of the smaller left valve; commonly a 

 small rounded spot present near the center of the valves. Geno- 

 type C. ovata Roemer. Ordovicic-Recent. 



117. C.(?) rugosa Jones. {Fig. 1666, t,f,ti,n'. ) Ordovicic. 

 Surface rather coarsely pitted. 



Black River of Canada; Trenton of Minnesota. 



118. C. ovatiformis Ulrich. (Fig. 1666, v-x.) Mississippic. 

 Nearly oval; surface smooth. 



Chester of Kentucky. 



119. C. marlboroensis Ulrich. (Fig. i66g,e-h.) Eocenic. 

 Very like the preceding, but slightly more regular ; surface finely 



pustulose. 



Eocenic (Pamunkey) of Maryland. 



XLIX. Metacypris Brady and Robertson. 

 Subrhomboidal shells, rounded in front, obscurely angular be- 

 hind; dorsal view heart-shaped in female, broadly ovate in male; 

 ventral surface deeply impressed along central and posterior por- 

 tions of median line; right valve sHghtly larger than left, with 

 hinge formed anteriorly by a laminated angular projection and 



