Maryland Geological Survey 639 



hinge-line, the wing less defined, and its posterior extremity not pro- 

 duced into a spine." Hall, 188-i. 



The individual?; referred to this species present considerable variation 

 in shape as shown by the figures. 



Occurrence. — Jexnixgs Formatiox, Chemung Member. National 

 Eoad 7 miles west of Frostburg; Oakland-Eedhouse Road; Allegany 

 Grove, 2030 common, 2215, 2307, 2325 common. 



Collection.' — Maryland Geological Survey. 



Leptodesma navieorme Hall 

 Plate LXIII, Figs. 16, 17 



Leptodesma imviforme Hall, 1884, Pal. of N. Y., vol. v, pt. i, sec. i, p. 200, 

 pi. xxii, fig. 15; pi. xxiii, fig. 1. 



Description. — "Shell below^ the medium size, rhomboidal; body very 

 oblique, short-ovate ; length more than one-third gi-eater than the height ; 

 anterior extremity subtruncate; margin curving forward, nearly vertical. 

 Left valve convex below, very gibbous above the middle. Right valve 

 depressed-convex below, gibbous above. Hinge-line straight, more than 

 one-third greater than the height of the shell. Beaks obstuse, situated 

 at the anterior fourth of the hinge-line, prominent, directed slightly 

 forward. Umbonal region gibbous, sloping abruptly into the wing. 

 Anterior end short, acute. Wing large, not distinctly limited, extending 

 almost to the posterior extremity; margins scarcely concave below; ex- 

 tremity produced, acute. Test marked hj fine concentric striae, which are 

 crowded into fascicles at nearly equal intervals, rounded upon the upper 

 part of the body, and subangular on the lower part. The strife are 

 crowded and lamellose on the anterior ; on the posterior slope they make a 

 short abrupt curve, passing over the wing with a gently forward direction 

 and. curving backward just below the hinge-line. In the weathered 

 surface of the right valve the concentric undulations are stronger, the 

 postcardinal slope is marked by strong interrupted radii which appear 

 to belong to the intimate structure of the shell. The hinge is marked 

 by a single narrow groove." Hall, 1884. 



Length 20 mm.; height 13 mm. 



