STROPHODONTiE OF THE CHEMUNG GROUP. HI 



knOAvn except in the condition of casts, the shell being rarely preserved in any 

 of the epecimens obtained. 



The figures 1 - 5 on Plate xix show the ordinary forms of casts of the ventral 

 valve ; no casts of dorsal valves in a condition to be illustrated, have been 

 found up to this time. 



Geological formation and localities. In the Chemung group, on Cayuta creek ; 

 near Elmira ; along the line of the Blossburgh railroad ; near Bath in Steuben 

 county, and in other localities in the southern counties of the State. 



Strophodonta nmcronata. 



PLATE XIX A. 

 Btrophodonta mucronata : Conrad, Jour. Acad. Nat. Sciences, Philadelphia, Vol.viii, p. 257. 1842. 



The description of Mr. Conrad is as follows : — 

 " Winged : inferior valve convex-depressed, with crowded unequal 

 " radii, and very fine transverse wrinkles ; umbo flattened ; apex on 

 " a line with the hinge-margin ; hinge extremities acutely angulated." 

 The species which I have identified with this one is a semioval shell, 

 the proportions of length and breadth varying considerably, the hinge- 

 line often extended beyond the width of the shell. The ventral valve is 

 very depressed-convex, and the dorsal valve nearly flat. The area of the 

 ventral valve is extremely narrow, and the inner margin crenulated for 

 about half its length. The surface is marked by distant sharp elevated 

 striae, with the interspaces occupied by numerous finer undulating striae. 

 The muscular impressions are faintly marked ; those of the occlusor 

 muscles occupy a minute space near the apex of the cast ; and the diva- 

 ricator muscular impressions are elongate and spreading, and partially 

 limited by an oblique pustulose ridge on each side, in form and cha- 

 racter like those in S. perplana, but less strongly defined. The cardinal 

 process of the dorsal valve is slender, and supported at its base by an 

 oblique ridge on each side, between which the muscular markings are 

 faintly impressed. 



This shell has the general form and the tenuity of S. perplana; but the muscu- 

 lar markings are not proportionally as large in the ventral valve, and the cardinal 

 process of the dorsal valve is much more slender and less diverging. The surface 



