288 
PALEONTOLOGY OP NEW-YORK. 
in the centre or slightly dichotomizing : these are crossed by strong 
concentric lamellose striae, which become very conspicuous on the 
wing, where no radiating striae are observed. 
The only specimen seen is the interior surface of a large left valve. The strong 
duplicating ribs, and the scarcely oblique direction of the body of the shell to the 
hinge-line, with the large posterior wing, are distinguishing features. 
Fig. 8. The interior of the specimen described. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Schoharie. 
Avicnla textilis (n. s.). 
Plate LII. Fig. 9 & 10?; and Plate LIII. Fig. 2, 3, 5, 7 & 10. 
Body of the shell obliquely subovate; length about once and a half the 
height, becoming regularly convex from the base, gibbous in the middle, 
and gently depressed along the line of junction with the posterior wing: 
ventral margin very regularly and broadly curved ; hinge-line greatly 
extended : posterior wing long, nearly three times its greatest width, 
the extremity extending beyond the margin of the shell, the margin 
moderately sinuate. 
Surface marked by regular strong radiating ribs, which, at the base, are 
distant from each other three times their width ; the intermediate 
space marked by a central finer ray, and, on each side between it and 
the larger costae, are one or two still finer rays, which are scarcely 
perceptible to the naked eye : these are crossed by concentric ridges, 
giving a cancellated surface and a slightly nodose character to the 
larger costae. The wing is marked by strong radiating and concentric 
striae, which are of nearly equal size, and slightly nodose at their 
junction. 
The form of this shell, in its full-grown condition, with its strongly cancellated 
surface, sufficiently distinguishes it from any other species known to me in this 
group of strata. 
