108 



Contributions to Palaeontology, 



1850, I have been unable to find a Disctna ; and I have 

 been disposed to believe that the broad convex linguloid 

 form, so common in these beds, was at that time referred 

 to Orbicula. 



In all the collections I have but a single specimen which I refer, 

 with doubt, to Discina ) but this is so obscure as scarcely to merit 

 description. It is from the later beds of the formation, and in the 

 same association with Dilcelocephalus minnesotensis, at Mazamonia, 

 Wisconsin. 



DISCINA INUTILIS (n. s.). 

 PLATE I. FIG. 11. 



Shell small, elliptically subquadrate, concentrically stria- 

 ted : apex excentric, nearly marginal. 



The specimen appears to consist of both valves, which in the pres- 

 sure have slipped, a little out of place. My only reason for 

 indicating this obscure specimen, is, that Orbicula (Discina) 

 has been regarded as a common fossil of the Sandstone. 



GENUS OBOLELLA (Billings, 1861). 



" Generic characters. Shell ovate, circular or subquadrate, 

 " convex or plano-convex. Ventral valve with a false 

 *' area which is sometimes minute, and usually grooved 

 " for the passage of the peduncle. Dorsal valve either 

 " with or without an area. Muscular impressions in the 

 " ventral valve four ; one pair in front of the beak near 

 " the middle or in the upper half of the shell, and the 

 " others situated one on each side near the cardinal edge. 

 " Shell calcareous. Surface concentrically striated, 

 " sometimes with thin extended lamellose ridges. 



" In general form, these shells somewhat resemble 

 " Obolus, but the arrangement of the muscular im- 



