46 JBrae'iiopoda of the Cincinnati Group. 



regions more or less thickened within, and roughened by the crossing 

 of the vascular markings, which are scarcely visible on any part 

 within this zone. 



Surface of both valves ornamented by numerous fine, closely 

 crowded radiating striae, sometimes alternately larger and smaller, the 

 smaller ones being shorter and intercalated. Concentric lines are 

 sometimes visible with the aid of a magnifier. Some specimens show 

 imbricating marks of growth near the free margins. 



Length of a medium specimen, three fourths of an inch ; breadth, 

 an inch ; and convexity, one fourth of an inch. 



Found at Versailles, Weisburg, Kichmond, Freeport, Clarksville > 

 and other localities in the upper part of the Cincinnati Group. Its 

 range seems to be confined to the rocks from 600 to 800 feet above 

 low water-mark. It is common within this range. 



Semipronites suhtenta — (Cone ad, 1847). 



This species so closely resembles H. planumbona that one might be 

 classed as a variety of the other. The chief differences are as follows-: 



The striae ornamenting the valves are possibly coarser on the sub- 

 tenta than on the planumbona, and a few wrinkles on the valves di- 

 rected inward and backward from the edge of the area on each side of 

 the beak characterize the subtenta. 



Its range is the same as that of planumbona and it is specially abun- 

 dant at Richmond, Indiana. 



Strophomena plicata (James) of the Ohio Paleontology is a synonym 

 for this variety or species. 



Hemijironites nutans — (James, 1871). 



Shell subtrigonal, strongly concavo-convex, comparatively thick 

 and strong ; hinge about equaling the breadth of the valves. 



Dorsal valve remarkably convex, with a prominent anterior region, 

 somewhat like a mesial ridge near the margin ; umbonal region flat- 

 tened ; beak not distinct from the edge of the area ; area sublinear 

 and directed backward. Interior with short bifid cardinal process ; 

 sockets deep, subtrigonal and very oblique ; socket-ridges prominent, 

 thin, and continued obliquely forward and outward with an inward 

 curve, so as to form the lateral margins of the muscular impressions, 

 which are well defined, rather deep, strongly striated, and separated 



