378 Thirty-fifth Beport on the State Museum. 



joining the body of the shell at the posterior extremity ; margin 

 symmetrically concave ; extremity produced and acutely angular. 



Test marked by distant, sub-equal, lamellose elevated striae, with 

 finer intermediate stria?. The stronger striae are regular on the wing 

 and posterior slope of the body, and become irregular and fasciculate 

 on the anterior side and lower half. The hinge shows a narrow 

 striated ligamental area, and, apparently, a slender lateral tooth. 



A large left valve has a length of 44 mm., height 28 mm., and 

 hinge-line 42 mm. 



This form is more oblique, and the wing is longer and narrower 

 than in L. Orodes. 



Formation and locality. In the Upper Chemung group at Mans- 

 field, Tioga county, Pa. 



Leptodesma Orcus. 



Leplodesma Orcus, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell of medium size, sub-rhomboidal ; 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, slightly impressed at the sinus, nearly straight along the 

 base and regularly rounded posteriorly. Left valve symmetrically 

 and gently convex below, slightly gibbous above. "Right valve un- 

 known. Hfnge-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 byssal 

 depression ; extremity acute. Wing of medium size, joining the 

 body about three-fourths its length from the beak ; margin abruptly 

 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 striae, which are somewhat 

 fasciculate on the body and anterior part of the shell, and a little 

 more sharply elevated on the wing. 



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

 and hinge-line 24 mm. 



The form and proportions of body are quite similar to L. Lysander, 

 but it is wider on the posterior and narrower on the anterior; the body 

 more oblique to the hinge; the striae less sharply elevated, curving 

 more forward along the posterior slope, and more abruptly recurved 

 under the cardinal line. 



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

 Tioga county, Pa. 



