Maryland Geological Survey 633 



Occurrence. — Jennings Formation, Genesee Member. Near Cum- 

 berland; Wolfe Mill; Corriganville ; Parker farm near Burlington, West 

 Virginia. Woodmont Member, Naples Fauna. National Eoad, Polish 

 Mountain, east of Gilpin; National Eoad west of Tonoloway Ridge. 



Co//ec^iO/i.— Mar}'land Geological Survey. 



Genus ECTENODESMA Hall 



Ectenodesma birostratum Hall 

 Plate LXII, Figs, 8, 9 



Ectenodesvia birostratuon Hall, 1884, Pal. of N. Y., vol. v, pt. i, sec. i, p. 242, 

 pi. xxiii, figs. 27-30; pi. Ixxxiv, fig. 20. 



Description. — " Shell large ; body ovate, oblique ; height greater than 

 the length ; margin regularly curving from the base of the anterior wing 

 to the postbasal margin, where it is somewhat abruptly recurved. Left 

 valve regularly convex below, gibbous in the umbonal region, somewhat 

 arcuate, the point of greatest convexity being about the middle of its 

 length. Eight valve concave below, depressed-convex in the middle, 

 convex on the umbo. Hinge-line straight, much longer than the length of 

 the shell, and, in extreme specimens, more than once and a half greater 

 than the length of the shell. Beaks acute, anterior to the middle of 

 the shell, inclined forwai'd, and arching over the hinge-line. Umbonal 

 region gibbous, limited on the anterior side by a shallow, undefined sulcus, 

 and on the posterior side by the abrupt depression of the body, subtending 

 an acute angle. Anterior wing large, triangular; margin concave; 

 extremity produced to an acuminate extension. Byssal sinus shallow 

 and undefined. Posterior wing large, triangular, joining the body below 

 the middle of its height; margin concave; extremity produced into an 

 acute termination. Test of left valve marked with regular and even 

 radii which are rounded above, flattened and sometimes bifurcate below; 

 similar but more acute radii continue over the posterior wing. In the 

 right valve the rays are more numerous, finer and sharp, and are con- 

 tinued upon the posterior wing with a little less force than on the body 

 of the shell; and very much subdued upon the anterior wing; entire sur- 

 face marked by fine, even, concentric stride of gTowth. The specimens 



