Lamellibranchiata. 379 



LEPTODESMA L* SANDER. 



Leptodesma Lysander, Hall. Pal. N. Y., vol. v, pt. 1 . Plates and Explanations ; 

 PI. 22, fig. 22. Jan., 1883. 



Shell of medium size, sub-rhomboidal ; body ovate, abruptly nar- 

 rowing toward the front, oblique to the hinge-line at an angle of 

 about 45°; length one-third greater than the height; ante-byssal 

 margin oblique above, curving into the marked sinus, regularly 

 rounded along the base and posterior extremity. Left valve de- 

 pressed-convex below, more convex above, and moderately gibbous 

 on the umbo. Eight valve unknown. Hinge-line straight, about 

 equal to the greatest length of the valve. Beak sub- anterior, 

 moderately prominent. Umbonal region but slightly gibbous, sub- 

 tending an acute angle. Anterior end Woad, obliquely truncate ; 

 extremity slightly extended and acute. Wing narrow-triangular, 

 joining the body at the posterior extremity ; margin concave, great- 

 est concavity just above the middle, thence turning outward it is 

 produced into a spiniform extension of the cardinal line. 



Test marked by fine concentric striae, with sharp lamellae at nearly 

 regular intervals over the body and wing, becoming crowded and 

 fasciculate on the anterior of the shell. 



A specimen has a length of 35 mm., height 24 mm., and hinge- 

 line 33 mm. 



This species differs from L. Gory don in its shorter and broader form; 

 the body is less oblique and more abruptly narrowed toward the 

 anterior ; the striae are less abruptly recurved along the posterior slope. 



Formation and localities. In shales of the Chemung group at 

 Kelly's creek and Mansfield, Tioga county, Pa. 



Leptodesma Nereus. 



Leptodesma Nereus, Hall. Pal. N. Y. , vol. v, pt. 1. Unpublished, 



Shell of medium size, sub-rhomboidal ; body broadly ovate, narrow- 

 ing rapidly to the anterior end, and oblique to the hinge-line at an 

 angle of 45°; length less than one-third greater than the height ; 

 ante-byssal margin gently curved, extending into a wide sinus, 

 broadly rounded along the base, curving regularly on the post-basal 

 side and abruptly recurved in the upper part. Left valve gently 

 convex below, more convex above, and moderately gibbous on the 

 umbo. Eight valve smaller, more depressed below, nearly equally 

 convex toward the umbo. Hinge-line straight, less than the length 



