LAMELLIBRANCHIATA. 215 



Formation and locality. In the Upper Chemung group, Mansfield, Tioga 

 county. Pa. 



Leptodesma Orcus, n. sp. 



1>LATE XC. FIG^ -23 



Shell of medium size, sub-riiomboidal ; body ovate, straight on the posterior 

 slope, oblique to the hinge-line at an angle of about 40° ; length about one- 

 third greater than the height ; ante-byssal margin oblique, slightl}- impressed 

 at the sinus, nearly straight along the base and regularly rounded posteriorly. 



Left valve symmetrically and gently convex })elow, slightly gibbous 

 above. Right valve unknown. 



Hinge-line straight ; length equal to three-fourths the length of the shell. 



Beak anterior, acute, little elevated above the hinge-line. Umbonal region 

 narrowly gibbous, subtending a very acute angle. 



Anterior end short, limited by an oblique bys.sal depression ; extremity 

 acute. Wing of medium size, joining the body about three-fourths its length 

 from the beak ; margin alnniptly curving forward from the base to near the 

 hinge-line, where it is sharply recurved into a short, angular extension of 

 the cardinal line. 



Test thin, marked by fine concentric stria3, which are somewhat fascicu- 

 late on the body and anterior jiart of the shell, and a little more sharply 

 elevated on the wing. 



Interior unknown. 



The specimen illustrated has a length of 30 mm., height 22 mm., and 

 hinge-line 24 mm. 



The form and pi'oportions of body are quite similar to L. Lysander, but it 

 is wider on the posterior, narrower on the anterior end, and more oblique to 

 the hinge ; the stria? less sharply elevated, curving more forward along the 

 posterior slope, and more abruptly recui'ved under the cardinal line. 



Formation and locality. In the Chemung group at Kelly's creek, Tioga county. 

 Pa. 



