FOSSILS OF THE NIAGARA GROUP. 385 



GENUS CYPRICARDINIA, Hall. 



Pkobablt = SEDGWICKIA, M'Coy. 



Cypricardinia ARATA, N. S. 



plate xiv, fig. 6. 



Shell subovate, varying from moderately to extremely gibbous ; body of 

 the shell subovate, alate posteriorly ; beaks near the anterior end, 

 which is short and rounded. Surface marked by strong concentric 

 lamellose ridges. 



The species resembles in form the Modiolopsis (^Cypricardinia) undidos- 

 triata of the Niagara shale of New York ; but the concentric ridges are 

 stronger, and the fine undulating striae are not visible in any of the 

 western specimens. 



Formation and Locality. — In limestone of the Niagara group, at Racine, 

 Wisconsin, and Bridgeport, Illinois. 



GENUS MODIOLOPSIS, Hall. 

 Modiolopsis oicTiEus, n. s. 



plate XIV, FIG. 7. 



Shell broadly subovate, moderately convex, becoming somewhat gibbous 

 on the umbo ; beak about one-fifth distant from the anterior 

 extremity ; gradually expanding posteriorly so that half way between 

 the beak and the posterior margin it is once and a half as wide as 

 in the line just anterior to the beak. The straight hinge extends 

 about half the length of the shell, and the cardinal mai'gin is thence 

 gradually curved to the posterior end. The cast of the hinge-line 

 shows two narrow lateral folds or teeth. Surface concentrically 

 striated. 



The length of a specimen measured is about one inch, with the greatest 

 width seven-eighths of an inch. 



The form of this species is very similar to Modiolopsis modiolaris of the 

 Lower Silurian rocks ; but the hinge-line does not rise so abruptly on 

 the posterior side of the beak, and the contraction below the beaks is 

 not observed. 



Formation and Locality. — In limestone of the Niagara group, at Racine, 

 Wisconsin, and at Bridgeport, Illinois. 

 Cab. Nat. 49 



