282 PALEONTOLOGY OF ILLINOIS. 



margin regularly rounded, anterior edges of the two valves 

 rather abruptly inflected towards each other, from the suban- 

 gular, umbonal slopes; beak of left valve pointed, curving 

 obliquely forward and inward. Surface marked by slender, 

 rather obscure, regularly arranged, concentric lines, separated 

 by smooth spaces some eight or ten times as wide as the lines. 

 Height, 0.75 inch; breadth, 0.59 inch; convexity (of a left 

 valve), about 0.21 inch. 



The only specimens of this species yet discovered, so far as we have seen, 

 are left valves, which are rather distinctly compressed for a species of this 

 genus. Its small size, compressed form, and comparatively smooth surface, 

 will serve to distinguish it from all the other species with which we are 

 acquainted. 



Locality and position : Spergen Hill, near Salem, Indiana; from the horizon 

 of the St. Louis group of the Lower Carboniferous series. 



Genus YOLDIA, Moller, 1842. 



(Kroyer's Nat. Tid., iv, p. 94.) 



Yoldia? levistkiata, M. and "W. 



PI. 20, fig. la, lb. 



Leda (Yoldia?) levistriata, Meek and Worthen, Oct , 1860. Proceed. Acad. Nat. Sci., 

 Philad.,p. 457. 



Shell rather under medium size, transversely subovate, mod- 

 erately convex in the central and anterior regions ; base form- 

 ing a broad, semiovate curve, the most prominent part being a 

 little in advance of the middle; anterior side rounded; poste- 

 rior side compressed, narrower than the other, and narrowly 

 rounded at the extremity, which appears to be slightly gaping; 

 beaks depressed and located centrally, or a little behind the 

 middle ; dorsal outline declining slightly and somewhat con- 

 cave behind the beaks, a little convex in front ; surface appa- 

 rently smooth, but showing under a good lens extremely fine, 

 regular, closely arranged, concentric striae. (Hinge and 



