364 Thirty-fifth Report on the State Museum. 



This species differs from L. Hector in its greater obliquity, broader 

 body below, more produced and abruptly recurved post-basal extremity, 

 deeper and broader byssal depression, more produced anterior end, and 

 narrower and more deeply sinuate wing, with a more extended ex- 

 tremity. 



Formation and localities. In the central portion of the Chemung 

 group at Philipsburg, Alleghany county, N. Y., and below the con- 

 glomerate (same position) in Sullivan township, Tioga county, Pa. 



Leptodesma curvatum. 



Leptodesma curvatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 25, fig. 5. Jan., 1883. 



Shell above the medium size, rhomboidal; body ovate, sub-arcuate 

 above, oblique at an angle of nearly 30° with the hinge-line ; length 

 and height as 7 to 4 ; ante-byssal margin oblique above, curving 

 into a long, shallow sinus ; basal and posterior margins forming a 

 broad, continuous curve, and abruptly recurving at the post-basal 

 extremity. Left valve regularly convex below, gibbous above the 

 middle. Right valve unknown. Hinge-line straight ; length about 

 equal to the length of the valve, but not fully seen, as the spiniform 

 termination is imperfect. Beak sub-anterior, prominent, directed 

 forward. Umbonal region gibbous, subtending an acute angle. 

 Anterior end large, abruptly acute at the extremity, limited by a 

 distinct, nearly vertical byssal depression. Wing narrow-triangular, 

 joining the body at the posterior extremity ; margin very oblique 

 below, concave above, abruptly recurved just below the cardinal 

 line and extended into a spiniform process. In a cast of the left 

 valve the wing is distinctly separated from the body by a marked 

 furrow. 



Test moderately thick, marked by elevated lamellose striae, with 

 finer intermediate lines. 



The specimen described has a length of 42 mm., height 24 mm., 

 hinge-line, to base of spine, 30 mm. The spine is probably extended 

 from 5 to 10 mm. beyond. 



This species resembles L. lepidum, but differs in its more gibbous 

 form, larger and more obtuse anterior extremity. 



Formation and localities. In sandstones of the Upper Chemung 

 group, McKean county, Pa., on the road from Bradford to Farmers 

 Valley, Cattaraugus county, N. Y. 



