216 PALEONTOLOGY OF NEW YORK. 



Lei'Todesma Lysander. 



PLATE XXII, VIG.-2-2; and PLATE XC, FIG. .14. 

 L'pUxhuma Lysamhr, H.\i,i.. Pal. N. V.. vdl. \, jit. 1. Platc.^ a;iil Kxiil.-ination.s : PL 22. i\g. 22. Jan.. 1883. 



Shell of medium size, sul>rhomboidal ; body ovate, abruptly narrowing 

 toward the front, ol)li(|Uc to tlie hinne-line at an angle of al)out 45°; length 

 one-third greater tliiin the height ; ante-bys.'^al margin oblique above, curv- 

 ing into the marked ;<inu.><, regularly rounded along the base and po.sterior 

 extremity. 



Left valve depressed-convex below, more convex above, and moderately 

 gibbous on the uml)o. Right valve unknown. 



Hinge-line straight, about equal to tlie greatest length of the valve. 



Beak sub-anterior, moderately prominent. Umbonal region but slightly- 

 gibbous, subtending an acute angle. 



Anterior end broad, obliquely truncate ; extremity slightly extended and 

 acute. "Wing narrow-triangular, joining the body at the posterior extremity ; 

 margin concave, greatest concavity just above the middle, thence turning 

 outward it is produced into a spiniform extension of the cardinal line. 



Test marked by fine concentric strise, with sharp lamella? at nearly regular 

 intervals over the bod^ an<l wing, becoming crowded and fasciculate on the 

 anterior of the shell. 



Interior unkiiow n. 



A specimen has a length of ;>•") nnu., height '24 mm.. Miid hinge-line 

 33 mm. 



This species differs from L. Corydon in its shorter ami liroa(KT form ; tlie 

 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 Chennmg grou]i at Kelly's creek 

 and Mansfield, Tioga county, Pa. 



