668 GEOLOGY OF OHIO. 



surface markings are not nearly as distinct, and the anterior end is more 

 uniformly rounded. Of species occurring in the same beds, P. grandis is 

 much larger and has a convex basal line, while P. elliptica has a more 

 regularly curved outline, and is higher posteriorly. 



Formation and Locality : Upper beds of the Cincinnati group, 

 Waynesville, Ohio. 



Psiloconcha sinnata, n. sp. 



Plate 52. Figs.' 15 and 16. 



This species is associated in the same layers with P. inornata, but it 

 will be distinguished from that form at once by the unusual develop- 

 ment of the mesial depression, which is deep enough to produce a slight 

 sinuation of the ventral margin and gives a degree of definition to the 

 umbonal ridge that is not equaled in any other species of the genus. 

 The length .also is relatively greater than in any of the others, being 36 

 mm, in a specimen 13 mm. high. Comparing other features P. subrecta 

 has a differently shaped posterior outline, and P. inornata is more regu- 

 larly rounded in front. P. subdvalis seems to me to be the nearest, differ- 

 ing chiefly in its greater height • and much less developed mesial depres- 

 sion. None of the other species are near enough to require special com- 

 parisons. 



The general expression of the shell reminds greatly of certain 

 species of Cymatonota, but the absence of dorsal folds shows that it can- 

 not belong to that genus. 



Formation and Locality : Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. 



Psiloconcha tenur'str/ata, n. sp. 



Plate 52. Fig:. 10. 



Shell small, about 11 mm. in length, and 4.5 mm. in height ; valves 

 depressed convex, narrowly rounded in front, slightly oblique posteriorly 

 with the ventral margin nearly straight, and the dorsal margin posterior 

 to the beaks very gently arcuate and almost parallel with the basal line. 

 Beak very small, situated more than one-fourth of the entire length from 

 the anterior extremity; umbonal ridge and mesial depression scarcely 

 distinguishable. Except on the cardinal slope, the surface is covered 

 with exceedingly fine thread-like concentric lines, of which as many as 

 fifteen are to be counted in the space of 1 mm 



The narrowly rounded anterior end, and the exceeding fineness ot 

 the concentric surface markings are the distinguishing features. The 

 small size is probably also distinctive for the species. 



Formation and Locality: Near the base of the Cincinnati group, 

 having been found near low water mark in the Ohio river, at Covington, 

 Kentucky. 



