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 cast retaining the two valves in connexion. The foramen, 

 aa seen in this specimen, extends more nearly to the margin than is shown in 

 fig. 1 6, which is seen from the inside. 



Geological position and locality. In the Oriskany sandstone : Helderberg moun- 

 tains, Albany county. 



Ortliis liippai'ioiiyx. 



Plate LXXXIX. Fia. 1 - 4 ; Plate XC. Fia. 1 - 7 ; Plate XCI. Fia. 4 & 5 ; 



and Plate XCIV. Fio. 4. 

 Mrypa unguiformis : Conrad*. 



HipparionyT proximiu : Vanuxem, Geological Report of the Third District, 1843, p. 124, f. 4. 

 Orthis hipparionyz : Vanuxem [.'] ; ScuxuB, Palseontographica ( Dunker und Yon Meter) 



III. Band, pag. 217, tab. xl, f. 1 a, 6, 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. 



SuEFACE marked by fine subequal striae, which are often bifurcated and 

 become much stronger towards the margin of the shell : slight remains 



* I am at present unable to find Mr. CouHAn's description of this species, though cited by Mr. Vanuxem 

 and traditioi.a'.ly Ituown by that name among the geologists of New-York. 



