JUEASSIC FOSSILS. . 361 



subangular, and with a slightly flattened or concave space on the anterior 

 side, just below the apex of the beak. Umbonal ridge elevated, strongly 

 arcuate and subangular, the surface of the shell sloping rapidly to the pos- 

 terior and postero-cardinal margins. Anterior slope of the shell nearly or 

 quite vertical, or sometimes a little concave near the middle of the length. 



Surface of the shell marked by numerous strong but very irregular 

 varices of growth, which give to the shell an extremely rugose appearance. 

 Between the varices the surface is marked by fine lines of growth. No 

 radiating lines or other surface markings beyond those described can be 

 detected. Substance of the shell thick and strong. 



The shell is a strongly marked and very characteristic species, and 

 judging from the fragments in the hand specimens of rock must have been 

 an abundant form in a certain layer at the locality where collected. We 

 know of no American species of the genus having any close resemblance to 

 this one. 



Formation and locality. — In rocks of Jurassic age, associated with 

 Jurassic fossils, near Sun Dance Hills, Black Hills. 



Genus VOLSELLA Scopoli. 



VOLSELLA (MODIOLA) FOEMOSA. 



Plate 5, fig. 15. 



Modiola (Pema) formosa M. & H., Proc. Acad. Nat. Sci. Phil., Dec. 1861 p. 439. 

 Volsella formosa Meek, Smithsonian Check List Jurassic Foss. N. A., 1861, p. 28. 

 Volsella formosa M. & H., Pal. Upp. Missouri, p. 86. 



Shell ventricose, very elongate-ovate or subelliptical, widest across the 

 shell at the end of the cardinal line, slightly arcuate, and becoming more 

 curved with increased growth. Cardinal line rather more than half the 

 length of the shell, slightly curved ; beaks small, nearly terminal and 

 appressed; anterior end broad, extending but little beyond the beaks, and 

 sharply rounded below the middle of the width to the basal border, which 

 is slightly concave ; posterior margin convex, gradually declining from the 

 extremity of the hinge line to the postero-basal extremity, where it is sharply 

 rounded. Body of the shell highly convex, somewhat subangular along 



