660 



GEOLOGY OF OHJO. 



Formation and locality : In the uppermost beds of the Trenton 

 limestone on Reservoir hill near Frankfort, Kentucky. 



Genus ORTHODESMA, Hall and Whitfield. 



Orthodesma, Hall and Whitfield, 1875, Pal. Ohio, Vol. II. p. 93; Ulrich, 1894, 

 Pal. Minn., Final Rep. Vol. Ill, Geol. and Nat. Hist, Sur, Minn., p. 516. 



In the Minnesota report above indicated, I have restricted and rede- 

 fined this genus, and shown that it is related to Modiolopsis rather than to 

 Orthonota. So far as known, the genus is restricted to the I,ower Siluri- 

 an rocks in which it is represented by about fourteen species, three of 

 them occurring in the Trenton, the rest in the various beds of the Cin- 

 cinnati group. The greater number of the latter are new to science and 

 as I found room for only two of them in the present work, the remainder 

 will have to await some other opportunity for publication. 



Orthodesma subangulatum, n. sp. 



Plate 55, Figs. 21-23. 



This species is closely related to O. rectum, H. and W., the type of the 

 gemis, but may be distinguished by a number of minor differences, 

 chiefly in the matter of outline. The shell is more elongate, the posterior 

 height being less and only about one-third of the entire length. The 

 ventral margin is straighter and sinnate rather than convex, while the 

 central and dorsal outlines are more nearly parallel. The anterior end is 

 uniformly rounded instead of being oblique with the most prominent 

 point in the upper part. Finally the posterior margin is a little more 

 oblique. Of other differences we may mention that the umbones seem 

 to have been somewhat smaller and merely flattened instead of sulcate, 

 while the umbonal ridge is stronger and more curved. 



The Trenton species O. subnasntum {Modiolopsis subnasuta, Meek 

 and Worthen),is higher posteriorly, while O. curvatum, Hall and Whitfield 

 has a more rounded posterior end and more sinuate ventral margin. None 

 of the other species are near enough to require comparisons. 



Formation and locality : Upper beds of the Cincinnati group, Rich- 

 mond, Indiana. 



Orthodesma parvum, n. sp. 



Plate 55, Pigs- 19-20. 



Shell small, elongate, about 22 mm. long, 7 mm. high at the beaks, 

 and 7.5 mm. near the uniformly rounded posterior end; greatest thickness 

 subcentral, about 5 mm.; anterior end narrowly rounded, almost acute; 

 back straight, base straight in the middle, on the whole very gently con- 

 vex. Beaks small, not prominent, situated about one-fourth of the length 



