PALAEONTOLOGY OF IOWA. 493 



the vascular impressions small and margined by an elevated 

 ridge : cardinal teeth obsolete ; cardinal process small; area 

 indented by the bifurcate cardinal process of the opposite 

 valve. 



This small species is well marked by its strong shell, extremely arcuate form, re- 

 curved (sometimes) subauriculate hinge extremities, strongly striated area, which 

 usually presents the entire absence of foramen, and has two deep indentations on 

 each side of the central cardinal process of the ventral valve, which are filled by the 

 two horns of the cardinal process of the other valve. This feature is often conspicuous 

 when the area does not appear to have been worn. In a single individual, there is a 

 subtriangular opening in the base of the ventral area, and a circular foramen between 

 the forks of the cardinal process of the dorsal valve on one side, and the cardinal 

 process of the opposite valve on the other. 

 Fig. 1 a, b, c. Dorsal, ventral and profile view of a specimen : natural size. 



Fig. 2 a. Interior of ventral valve, showing the muscular and vascular impressions, the 

 central cardinal process, and the indentations of the area made by the car- 

 dinal process of the opposite valve. 



Fig. 2 b. A portion of the same enlarged, showing the parts described. 



Fig. 2 e,f. Enlargement of the stria; from two specimens, showing in 2/ the prevailing 

 character, while some specimens present the features of 2 e. 



Geological formation and locality. In shales of the age of the Hamilton 

 group : Lime creek above Rockford, Iowa. 



Strophodonta lepida (n. s.). 



PLATE III. FIG. 3 a. b, c. 



SHELL semielliptical or subquadrate, broader than long ; 

 hinge line scarcely equalling the width of the shell. Ventral 

 valve moderately convex, somewhat flattened towards the 

 hinge and more abruptly curving in front, the greatest 

 elevation being somewhat transversely along the middle. 

 Dorsal valve moderately and uniformly concave : area 

 principally confined to the ventral valve, striated longitudi- 

 nally. 



SURFACE smooth, or marked only by a few imbricating 

 lines of growth. 



Interior of ventral valve marked by strong elevated spaces 

 for the attachment of muscular impressions, with broad fla- 

 belliform and bilobate vascular impressions, which are limited 

 above by a thickened margin. Interior of dorsal valve showing 



