348 (iEOLOGY OF THE IJLACK HILLS. 



LAMELLIBRANCHIATA. 



OSTREIDJi:. 

 Genus OSTREA Linn. 



OSTREA STRIGILECULA. 



Plate :i, figs. 8-12. 



Grypluca calceola var. nebracensis M. & II., Pal. llpp. Missouri, PI. 3, fig. 1 a-f. 



Not Grijphcca calceola var. nehracemis M. & H., loc. cit., p. 7.5, figs. A-E. 



Gryphoi calceola var. nebracensis (Sli. & H.) H. & W., Geol. Exi)l. Fortieth l^arallel, p. 



2GS, PI. 7, flg. 11. 

 Ostrea strigilecula White, Geol. Surv. lOOtli Meridian, etc. 



Shells of moderate size or smaller, very variable and irregular in form, 

 but usually somewhat cm-ved or subreniform in outline. Lower valve often 

 attached to foreign bodies by nearly its entire external surface, and when 

 separated shows a broad, irregular cicatrix, from the edge of which the 

 sides and basal margins rise at a considerable angle, and form the cup of 

 the valve of greater or less depth The posterior or cardinal portion of the 

 valve is often quite shallow, while the basal or front border rises to a height 

 equal to more than two-thirds of the' entire length of the valve. Cardinal 

 end of the valve narrow, triangular, and with a ligamental area of propor- 

 tionally large size. Upper valve flattened or very depressed- convex. Sur- 

 face of the valves strongly lamellose or concentrically ribbed and wrinkled ; 

 muscular impression large in most examples. 



The shells of this species are extremely variable in character, it being 

 quite difficult to select two specimens of the same form from a collection of 

 a moderate number. The shell has been referred to Gryphaa; but, as has 

 been remarked under the observations on the species in vol. iv, Geologi- 

 cal Exploration Fortieth Parallel, p. 287, it presents, at many localities, 

 the features of a true Oyster, and not those ascribed to the genus Gryphcea. 

 Since those remarks were written. Dr. C. A. White has described it under 

 the name Ostrea strigilecula, considering it entirely a distinct species from 

 the forms given by ^Icok and Ilayden, Paleontolog}' of the Upper 



