CKETACEOUS FOSSILS. 417 



an<,^iil;ir form of the shell and gibbous umbones will serve as distinguishing 



marks. We have some hesitation in referring this form to Dr. Morton's 



Ci/fherea missouriana simj)!}' from the fact that his figure above cited is 



somewhat more distinctly triangular than any of the specimens we have 



seen, and their outline would correspond rather better with that of Astarte 



evansi = Crassafella evansi H. & M , but the valves are not represented 



quite as vBntricose as is the case with that shell, and it is scarcely possible 



that Dr. Morton would have referred a shell of that character, as perfectly 



preserved as his figure would indicate, to the genus Cytherea. 



Formation and locality. — In limestone of the Fort Pierrd Group, on the 



Cheyenne River, ten or fifteen miles west of the mouth of Beaver Creek, 



Black Hills. 



Genus THETIS Linn. 



THETIS CIECULAEIS. 



Plate 11, figs. 22 to 24. 



Venus ? circularis M. & H., Pioc. Acad.. Nat. Sci, Phil., Nov. 1856, p. 8. 



Cyclhia f circularis ]\Ieek, Smithsonian Check List Invert. Foss., p. 13. 



Thetis f circularis (M. & H.) Meek, Pal. U. S. Geol. Surv. Terr., p. 190, PI. 17, tig. 8. 



Shell small and very thin, subcircular and subglobose, with propor- 

 tionally large, elevated, pointed, and incurved beaks, which are directed 

 obliqueh' forward, and situated a little in advance of the middle of the 

 valve ; border of the valve circular, except at the postero-basal margin, 

 which is slightly extended and obtuse ; hinge line very arcuate ; ligaments 

 small and rather deeply seated, owing to the prominence of the back of 

 the umbones of the valves Surface marked only by fine concentric lines 

 of growth, and when entire and in good condition appears to have been 

 quite smooth. Muscular imprints faintly marked, the anterior most distinct 

 and submarginal ; pallial line deeply sinuate, the sinus narrow, angular, 

 and directed obliquely upward toward the apex of the valve ; lobes of the 

 pallial line digitate. 



The small size of the shell and its rotund and gibbous form will readily 

 distinguish it from any other species. 



Formation and locality. — In beds of the Fort Pierr^ Group, near the 

 forks of the Cheyenne River, and near the mouth of Rapid Creek, Black 

 Hills, Dakota. 

 27 B H 



