THE LOWER SILURIAN ROCKS. 667 



Psiloconcha inomata, n sp. 



Plate 52. Figs. 11 and 12. 



Shell'about 30 mm. long, by 12.7 mm. high in the middle, with regu- 

 larly rounded, subequal ends, very slightly convex ventral and more 

 strongly arcuate dorsal margin. Beaks very small, umbonal ridge and 

 mesial depression scarely distinguishable. Surface markings concentric, 

 very obscure, the surface appearing almost smooth in most cases. 



This shell is closely related to P. subovalis but does not attain as great 

 a size, is relatively longer, and has a smoother surface and more regularly 

 rounded anterior margin. The last difference is the most important and 

 may always be relied upon. 



Formation and Locadty. — Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. 



Psiloconcha elliptica, n sp. 



Plate 52. Figs. 3 and 4. 



Shell 30 mm. to 40 mm. in length, and a very little more than twice 

 ?s long as high; thickness less than half the height. Outline elongate 

 ovate; the regularity and neatness of the curves being a striking feature ; 

 yet when examined the posterior half proves distinctly wider than the 

 anterior, the height at the beaks being one-eighth or one-ninth less than 

 the greatest posterior height. v Beaks very small, situated 8 mm. behind 

 the anterior extremity in a specimen 38 mm. long; umbonal ridge just 

 appreciable. Surface nearly smooth, marked with mostly distant con- 

 centric lines. 



Formation and Locality. — Upper beds of the Cincinnati group, Clarks- 

 ville, Ohio. Several casts of the interior from Richmond, Indiana, may 

 belong here, but for the present I am obliged to consider them as doubtful. 



Psiloconcha subrecla, n. sp. 



Plate 52. Figs. 13 an 1 14. 



Shell about 32 mm. long, 12 mm. high, and nearly 6 mm. in thick- 

 ness"; cardinal margin gently arcuate, posterior end gaping widely, regu- 

 larly rounded in outline; basal line nearly or quite straight in the central 

 part; anterior end straight on the upper side, then turning abruptly 

 downward, the rest curving regularly like the posterior margin. Beaks 

 very small, situated a little less than one-sixth of the length of the shell 

 behind the' anterior extremity ; umbonal ridge moderate, mesial depress- 

 ion just appreciable. Surface of the cast with moderately distinct, sub- 

 equal lines of growth, very obscure on the cardinal slope. 



In this species the anterior end is shaped as in P. subovalis, but the 

 lengths proportionally too great for that species. In P. inomata the 



