988 PALiEONTOLOGY OF NEW-YORK. 



in the centre or slightly dichotomizing : these are crossed by strong 

 concentric lamellose strite, 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 slialy limestone of the Lower Helderberg 

 group : Schoharie. 



Avicula textilis ( n. s.). 



Platb LII. Fia. 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, suflBiciently distinguishes it from any other species known to me in this 

 group of strata. 



