ORISKANY SANDSTONE. 
407 
This species resembles the Discina discus of the Lower Helderberg rocks; but it 
is much larger than any specimen of that species observed, the concentric lamellae 
are much stronger, and the specimens (which are principally casts or impressions 
of the exterior) do not show radiating striae. 
Fig. 1 a. The dorsal valve, from a cast taken from the mould of the original in sandstone. 
Fig. 1 b. The ventral valve from the inside, the shell having been mostly removed by wea¬ 
thering. 
Fig. 1 c. Profile of the dorsal valve. 
Fig. 1 d. The ventral valve, from a east retaining the two valves in connexion. The foramen, 
as seen in this specimen, extends more nearly to the margin than is shown in 
fig. 1 b, which is seen from the inside. 
Geological position and locality. In the Oriskany sandstone : Helderberg moun¬ 
tains, Albany county. 
Orthis hipparionyx. 
Plate LXXXIX. Fig. 1 - 4 ; Plate XC. Fig. 1 - 7 ; Plate XCI. Fig. 4 & 5 • 
and Plate XCIY. Fig. 4. . 
Atrypa unguiformis : Conrad*. 
Hipparionyx proximus : Vanuxem, Geological Report of the Third District, 1843, p. 124, f. 4. 
Orthis hipparionyx : Vanuxem[?] ; Schnur, Palseontographica ( Dunker und Vox Meyer), 
HI. Band, pag. 217, tab. xL, f. 1 a, b, c. 
Shell suborbicular or depressed-hemispheric; length and width nearly 
equal, or varying from this form to that of length and width as seven to 
eight (In one specimen, having a width of three inches and one six¬ 
teenth, the length is one-eighth of an inch less than the width). Dorsal 
valve regularly convex, the greatest convexity near or a little above 
the middle of the shell, and sloping evenly towards the sides and front; 
the margin, towards the dorsal line, a little more compressed : beak 
prominent, incurved. Ventral valve nearly flat, a little convex near 
the beak, and often slightly concave towards the front : area wide, of 
moderate height, flat; foramen usually closed or partially closed by a 
deltidium. 
Surface marked by fine subequal strios, which are often bifurcated and 
become much stronger towards the margin of the shell : slight remains 
* I am at present unable to find Mr. Conrad’s description of this species, though cited by Mr. Vanuxem 
and traditionally known by that name among the geologists of New-York. 
