STROPHODONTiE OP THE CHEMUNG GROUP. HI 
known except in the condition of casts, the. shell being rarely preserved in any 
of the specimens 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 mucrpnata. 
PLATE SIX A. 
Strophodonta 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 
