JURASSIC FOSSILS. 359 



length of the body of the shell, measuring from the beak to the base with 

 the obliquity of the valve. Anterior ear very small and barely perceptible; 

 beak not rising- above the cardinal line, small, and somewhat compressed ; 

 outer margin of the posterior wing nearly at right angles to the hinge line 

 for some distance below, then directed backward to the extremity of the 

 basal line ; anterior margin very oblique to the hinge and parallel to the 

 body of the shell 



Having only a single right valve of the species, and that of small size, 

 it is somewhat difficult to satisfactorily identify it ; but there appears no 

 feature on the shell that would contradict the reference. 



Formation and locality. — In sandy limestone of Jurassic age, associated 

 with Ostrca strigilecula and Mytilus tvliitei, near Sun Dance Hills, Black 

 Hills. 



AROIDiE. 



Genus GRAMMATODON M. & H. 

 GRAMMATODON INORNATUS. 



Plate 5, figs. 16-18. 

 Area (Ciiculhea) inornata M. & H., Proc. Acad. N. Sci. Phil., March, 1858, p. 51. 

 Grammatodon inornatus M. & H., ibid., Oct. 1862, p. 419. 

 Qrammatodon inornatus M. & H., Pal. Upp. Missouri, p. 90, PL iii, fig. 9. 



Shell small, arciform, subrhomboidal in outline, with ventricose, arcu- 

 ate valves, sometimes quite gibbous on the umbonal region. Valves a little 

 less than twice as long as high between the cardinal and basal borders ; 

 hinge line straight, somewhat shorter than the shell below ; anterior end 

 rounded, extending a little beyond the antero-cardinal extremity ; posterior 

 end obliquely truncate, the longest point being at the extremity of the 

 posterior umbonal ridge ; basal line forming a broad semielliptical curve, 

 being most strongly rounded at each end. Beaks large, strong, slightly 

 incurved, strongly projecting above the body of the valve, subapproximate, 

 and situated just behind the anterior third of the length of the shell ; pos- 

 terior umbonal ridge somewhat angular, most distinctly so in the upper 

 part, becoming more obtuse below ; posterior cardinal slope abrupt and very 

 gently concave between the ridge and the cardinal line ; anterior umbonal 

 region rounded. 



