Maryland Geological Survey 457 



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 costse, 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 costse. 

 The wing is marked by strong radiating and concentric strice, which are of 

 nearly equal size, and slightly nodose at their junction." Hall, 1859. 



Length 4.7 cm. ; height 4.5 cm. 



Occurrence. — Helderberg Formation, N"ew Scotland Member. 

 Cumberland, Dawson. 



Collections. — Maryland Geological Survey, George M. Eoeder. 



[Ohern.] 



ACTINOPTERIA TEXTILIS VAR. ARENARIA (Hall) 



Plate LXXVII 



Avicula textilis var. arenaria Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 4C5, 



pi. cix, figs. 1, 2; pi. ex, fig. 2, 1861. 

 Avicula textilis var. arenaria Weller, 1903, Pal. N. J., vol. iii, p. 360, pi. 1, 



fig. 1. 



Description. — " 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, showing 

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

 by strongly elevated imbricating lamella?." Hall, 1859. 



