ORISKANY SANDSTONE. 4«« 



LAMELLIBRAXCHIATA OF THE ORISKANY SANDSTONE. 



The lamellibranchiate shells of this rock comprise few species, and these 

 usually occur in such condition as to afford very unsatisfactory means of 

 determination ; the specimens being for the most part in the form of casts 

 of the interior, or impressions of the exterior of the shell. 



ATicula textilis, var. arcnaria. 



Plate CIX. Fig. I & 2; and Plate CX. Fig. 2. 

 Avicida textUis : This volume, page 288. 



Shell large, obliquely subovate ; the proportions of length and height 

 variable. Left valve becoming moderately and regularly convex from 

 the base, the greatest convexity being about the first third below the 

 hinge-line. Posterior wing large, extending along the margin of the 

 body of the shell halfway from beak to base. Anterior wing small, 

 triangular, wrinkled. The right valve is slightly concave, smaller than 

 the other, faintly marked by the radiating ribs, which sometimes are 

 scarcely seen. 



Surface marked by strong radiating ribs sometimes regularly dichotomizing 

 and subequal, and in other specimens quite unequal, skowing a few 

 stronger ribs with several finer ones between, and these are crossed by 

 strongly elevated imbricating lamellae. 



This is the common form of Jlvicula in the Oriskany sandstone, and often attains 



a height of four inches or more. Although presenting considerable variety in its 



aspect and surface markings, I am unable at the present time to point out reliable 



specific distinctions between this one and the species of the Lower Helderberg 



limestones. 



PLATE CIX. 



Fig. 1. An impression made in sandstone by the exterior surface of the smaller or richt 

 valve. 



Fig. 2. The exterior imprint of the left valve of this species. 

 [ Paleontology III.] 69 



