350 Thirty-fifth Report on - the State Museum. 



acute angle. Anterior end short, defined by the oblique byssal 

 depression which extends to more than one-third the length of the 

 valve below the beak ; extremity obtusely angular. . Wing not large, 

 joining the body nearly at the posterior third ; margin oblique 

 below, acutely recurved above, the greatest concavity being just 

 below the cardinal extension ; extremity produced into a long spini- 

 form process. 



Test thin, marked at irregular intervals by strong concentric 

 lamellae with intermediate fine striae. The lamellae are stronger on 

 the anterior side and obscure on the wing. The casts are often 

 nearly smooth. 



A specimen of medium size has a length of 29 mm., height 20 

 mm., hinge-line, to base of spine, 12 mm. A smaller specimen has 

 a length of 20 mm., height 14 mm., hinge-line, to base of spine, 

 12 mm. 



This species closely resembles L. Roger si, but is distinguished by 

 the oblique and more extended byssal depression, shorter wing, and 

 more rounded basal margin. 



Formation and localities. In shales of the Chemung group, on the 

 Chemung river above Elmira, and along the Blossburg railroad near 

 Painted Post, N. Y. 



Leptodesma LONGISPI2STUM. 



Avicula longispina, Hall. Geolog. Surv. N. Y. : Rep. Fourth Dist., p. 262, 

 fig. 3. 1843. 



Pterinea longispina (Hall), S. A. Miller. Cat. Amer. Pal. Foss. 1877. 



Leptodesma longispinum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Ex- 

 planations : PI. 21, figs. 14, 17-19. Jan., 1883. 



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

 very oblique ; length greater than the height, often nearly double ; 

 margin from the byssal sinus to beyond the base broadly rounded ; 

 posterior margin extended and abruptly recurved. Valves, in 

 young specimens, sub-equally convex ; in older specimens the right 

 valves are somewhat less convex. Hinge-line straight; length 

 equaling or greater than the length of the valve. Beaks anterior, 

 prominent, obtuse, nearly erect, arching over the hinge-line. Urn- 

 bonal region gibbous, subtending an acute angle. Anterior 

 extension scarcely auriculate, rounded in front, straight above. 

 Wing small, very narrow-triangular ; margin deeply sinuate, the 

 bottom of the sinus being close beneath the hinge-line ; extremity 

 produced into an elongate spine. 



