THE LOWER SILURIAN ROCKS 649 



ckia as a departure from Clionychia during the Cincinnati epoch. The 

 radially ribbed, byssiferous genera having been established either pre- 

 viously or at the same time, it follows that ' Psilonychia cannot stand in 

 genetic relationship to them. 



Although I am acquainted with two species having the characters of 

 this genus, only one can be described at this time, the specimen of the 

 other being too imperfect for satisfactory delineation. Though so imper- 

 fect, it yet preserves all the essential characters of the genus. So far as 

 comparison between it and P. perangulata is possible, the undescribed 

 species appears to differ in being more oblique, and in having the anter- 

 ior and umbonal regions strongly rounded instead of angular. The 

 ■specimen is also from a higher horizon, having been found at Clarksville, 

 Ohio, in the upper beds of the Cincinnati group. 



Psilonychia perangulata, n. sp. 



Plate 51, Figs. 1—3. 



Shell large, high, accuminate-subovate, very slightly alate, rather nar- 

 now above, the lenght of the hinge line equally only about five-eights of 

 the greatest width of the shell; greatest width andgreatest height (from 

 umbo to center of base) about as three is to five; cardinal outline 

 straight in the posterior half, anteriorly rising to the umbo; anterior side 

 almost flat and at right angles to the hinge line in the upper half, curv- 

 ing backward below; lower half of outline semiovate. Beaks very prom- 

 inent, strongly incurved, carinate, the flattening of the anterior side 

 extending to their apices. Byssal opening large, situated about its length 

 beneath the beaks. Surface of test covered with concentric lines of 

 growth. These are rather regular and fine, except in the vicinity of the 

 byssal opening where they are gathered into distinct wrinkles. 



Byssonychia richmondensis , though a smaller shell, has a similar form, 

 but as it is restricted to strata between three and four hundred feet above 

 the beds holding P. perangulata, and as the Byssonychia has strong radiat- 

 ing ribs, which are wanting entirely in the species just described, there is 

 not the slightest occasion for confusion between them. 



Formation and locality: Middle beds of the Cincinnati group, near 

 the tops of the hills at Cincinnati, Ohio. 



Genus ANOPTERA, n. gen. 



Shell subovate, high, moderately convex, with a very short hinge, 

 not alate posteriorly; beaks terminal, directed forward, compressed; a 

 long, lunule-like depression beneath the beaks; no byssal opening. Sur- 

 face marked with concentric lines of growth only. Hinge apparently 

 edentulous; so far as known, no external ligamental area, but an inter- 

 nal cartilage, extending nearly the full length of the hinge, is indicated 



