8 The Faux a of the Keysee Member of the 



Description. — " Shell semioval, varying in form from length and width 

 equal to length greater or less than the width: hinge-line equal to or 

 greater than the width of the shell below; extremities rounded or salient. 

 Dorsal valve flat, or more or less concave according to the convexity of 

 the ventral valve, but not conforming entirely to the curvature of the 

 latter. Ventral valve varying from slightly convex to gibbous, and some- 

 times abruptly arching towards the front; umbonal region more or less 

 prominent; beak usually a little elevated. Area narrow, almost linear. 

 Foramen linear or none. 



" Surface often finely and evenly marked with straight or slightly un- 

 dulating striae; more often with prominent sharp striae at more or less 

 equal distances from each other, and the intermediate spaces by minute 

 equal stria; and again in other specimens by alternating larger and 

 smaller striae, of which there are frequently three regular gradations in 

 size. Eadiating striae crossed by fine concentric elevated lines, and often 

 by undulations or indentations which are more conspicuous on those shells 

 where the striae are in fascicles of finer between stronger ones. Vascular 

 impressions of the ventral valve circumscribed by lamellae, more or less 

 distinctly flabellate: impressions of adductor muscles elongate-oval." 

 Hall, 1859. 



Length 1.1 cm. to 1.9 cm.; width 1.5 cm. to 2.5 cm. 



Occurrence. — Helderberg Foraiatiox, Iveyser Member. Cash 

 Valley, Maryland; Hyndman, Pennsylvania. 



Collection. — Maryland Geological Survey. 



Stropheodonta (Leptostropiiia) bipartita (Hall) 

 Plate LVIT, Figs. 17, IS 



Leptana Upartita Hall, 1858, Pal. N. Y., vol. ii, p. 326, pi. Ixxiv, figs. 4, 5. 

 Stropheodonta nearpassi Schuchert, 1897, Bull. U. S. Geol. Survey, No. 87, 



p. 427. 

 Stropheodonta textilis Schuchert, 1897, Bull. U. S. Geol. Survey, No. 87, p. 



427. 

 Strophomena (?) bipartita Schuchert, 1897, Bull. U. S. Geol. Survey, No. 87, 



p. 429. 

 Stropheodonta bipartita Weller, 1903, Pal. N. J., vol. iii, p. 226, pi. xx, figs. 1-5. 



Description. — " Shell with thin, nearly fiat brachial valve and slightly 

 convex pedicle valve, longitudinally subsemielliptical in outline, the 



