THE LOWER SILURIAN ROCKS. 661 



of the shell behind the anterior extremity; umbones and sides of valves 

 flattened; umbonal ridge rather distinct though not angular. About 

 midway between the umbonal ridge and cardinal margin an obscure im- 

 pressed line. Surface with comparatively coarse concentric lines, 

 strongest and regular on the cardinal slope, faint anteriorly. 



The point of greatest convexity is situated farther forward than usual 

 in this genus but in other respects this small species seems to be a true 

 Orthodesma. Of described species it is probably nearest O. rectum, H. & 

 W., but as it is much smaller and not so high posteriorly, and has a more 

 evenly rounded posterior extremity it is not likely to be confused with 

 that species. 



Formation arid locality. Middle beds of the Cincinnati group, Cin- 

 cinati, Ohio, where it occurs about 400 feet above the bed of the Ohio 

 river. 



Genus CYMATONOTA, n. gen. 



Orthonota (part. ), Conrad, 1841, N. Y. Ann. Geol. Rep., p. 51; Hall 1847, Pal. 

 N. Y., Vol. I, p. 299. 



Orthodesma {part.), Hall and Whitfild, 1875, Pal. Ohio, Vol. II, p. 93. 



Elongate solen-like shells, gaping more or less at both ends, with the 

 hinge line long and extending in a straight line anterior and posterior to 

 the small beaks; ventral and dorsalmargins subparallel. Hinge 

 plate very thin, edentulous; valves united by a delicate linear ex- 

 ternal ligament seemingly extending the full length of the hinge. Test 

 very thin, marked externally with fine concentric lines, and on each side 

 of the hinge line by short wave-like furrows. Pallial line and muscular 

 scars so faintly marked that even in the best preserved specimens they 

 can not be made out with certainty. 



Type: Cymatonata typicalis, n sp. 



Species of this genus were first referred to Orthonota, Conrad, and 

 later to O? thodesma, Hall and Whitfield. 



The first of these genera is now generally used and I believe justly 

 so, for upper Silurian and Devonian species only. I admit however, that 

 the proposed genus is in many respects like Orthonota, yet I would main- 

 that they are not identical. The ornamentation which usually pertains to 

 the species of Conrad's genus is wanting in Cymatonota. The ligiment 

 is believed by Hall to be internal in Orthonota whereas it is clearly exter- 

 nal in the earlier types here described. Finally, in the absence of any 

 statement to the contrary, we must believe that the margins of the valves 

 do not gape in Orthonota while they do at both ends in Cymatonota. 



The relations to Orthodesma are doubtful. That genus is, I am fully 

 convinced, closely allied to Modiolopsis and Whiteavesia, but I cannot say 

 this of Cymatonota. On the contrary the last genus seems to me to be 

 widely different and my arrangement of the group as a member of the 



