THE LOWER SILURIAN ROCKS. 669 



Psiloconcha minima n. sp. 



Plate 52. Figs. 8 and 9. 



Shell small about 10.5 ram. high at the beaks, rather strongly con- 

 vex in the umbonal region when compared with the other species. Out- 

 line narrowly elliptical ; anterior end a little wider than the posterior, 

 and more regularly rounded ; dorsal margin arcuate, base more gently 

 convex. Beaks comparatively prominent, full, strongly incurved, situ- 

 ated about one third of the entire length from the anterior extremity. 

 Umbonal ridge moderately developed, rounded ; mesial depression 

 scarcely distinguishable. The specimen described, which is a well pre- 

 served cast of the interior, is perfectly smooth. Anterior muscular star 

 faintly defined, relatively large but occupying the same position and of 

 about the same form as in P. grandis and P. elliptica. 



The prominence and less anterior position of the umbones, and the 

 small size of the shell', are the distinctive features. 



For?natio?i and Locality : Lower beds of the Cincinnati group, Cov- 

 ington Kentucky, where it was obtained in a layer about 150 ieet above 

 the bed of the Ohio river. 



Family CYRTODONTIDiE Ulrich. 

 Genus ORTONEIXA, n. gen. 



Cypricardiies, S. A. Miller, 1874. Ciu. Quart. Jour. Sci., vol. i, p. 147; also [part.) 

 1892, North Amer. Geol. and Pal., p. 476. (Not of Conrad.) 



Shell subquadrate, highest posteriorly, equivalve, very inequilateral, 

 with moderately prominent beaks and umbonal ridge. Surface with con- 

 centric lines of growth. Hinge as in Cyrtodonta, Billings, excepting 

 that the cardinal teeth .are relatively stronger and placed immediately 

 beneath the beaks. A well defined lunule and escutcheon present. 

 Adductor muscular scars subequal, the posterior one very faintly im- 

 pressed, ovate, and situated just beneath the posterior extremities of the 

 lateral teeth, the anterior one very deep, sharply defined on the inner 

 and upper sides by a clavicular ridge extending obliquely backward from 

 the hinge plate, Pallial line simple, distinct. Small pedal muscle 

 attached to the under side of the hinge plate immediately over the ante- 

 rior adductor scar. Casts of the interior marked by an oblique umbonal 

 sulcus. 



Type: Cypricardiies hainesi, S. A. Miller. 



Although on the whole much like Cyrtodonta, this genus is still 

 quite distinct and easily separated. The cardinal teeth are different as 

 stated above, and the lunule is a feature unknown in that genus. Then 

 the anterior muscular scar is more deeply impressed an 1 defined on the 



