FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 201 



The surface is marked by very peculiar radii, which, apparently, run from 

 the basal margin to the dorsal one, across the valves, but which make, near the 

 dorsal line, a rapid deflection into the direction of the beaks. These radii are 

 low and flat, and have a faint, but plainly observable depressed line in their 

 middle, a feature which I have never noticed in any other shell. Their inter- 

 spaces are large, from three to four times of their own width ; in some of 

 these interspaces there is a smaller intermediate line. This species has the 

 general features as Clinopistha subnasuta and antiqua, except its peculiar sur- 

 face-markings, and its greater size. I place it into the genus Clinopistha, on 

 account of its general form agreeing with that of C. subnasuta ; whether or 

 not interior characters will leave it there or transfer it to another known 

 genus, or make a new one for its reception, I can not say. The two specimens 

 illustrated on plate 4 are of average size, and are, as far as I know, the only 

 two shells of this species so far found. Both belong to my own collection. 



Formation and Locality. Found in the cherty layers overlying tile hydraulic limestone of the 

 Devonian formation, in Jeflerson county, Ky., and in Clark county, Ind. 



Genus Ptychodesma. Han. 



Ptychodesma, Hall. 24th Regent's Report 1872. 

 Etymology : ptychos, a folding ; desma, a ligament or band. 



Shell modioloid in form ; valves equally convex ; hinge with a wide liga- 

 mental area, the sides of which are sharply grooved in parallel lines, caused 

 by the successive growth of the ligament, as in Pectunculus. The grooves and 

 ridges are slightly arched beneath the apex of the valves, where they take 

 their origin. 



The internal hinge structure is unknown. In general form and characters 

 this genus resembles Modiomorpha, but differs in having a ligamental area 

 marked by fine striae parallel to the hinge-line, while this is parallel to the 

 margin of the shell. 



The type of the genus is Ptychodesma knappiana. 



Ptychodesma knappiana. HALL. 



Plate II., figures 13, 15, 16, 17 and 18. 



Ptychodesma knappiana, H. and "W. 24th Rep. N. Y. 1872. 

 Ptychodesma knappiana, Hall. Pal. N. Y., Vol. 6, pt 1, p. 3531885. 



Shell of medium size, obliquely sub-ovate ; length usually more than one- 

 third greater than the height ; basal margin oblique, often nearly straight in 

 the middle of its length. Posterior end broadly rounded, 



GEOL. SUR. 26 



