228 PALEONTOLOGY OF NEW YORK. 



region indistinctly defined anteriorly ; the posterior side limited by the post- 

 umbonal depression. 



Anterior end short ; extremity angular. Wing undefined, joining the 

 body one-third of the length from the posterior extremity ; margin gently 

 concave ; extremity scarcely produced. 



Test marked by fine, sharply elevated, concentric striaj, which are more 

 crowded anteriorly, and very regular on the wing. 



Interior unknown. 



The specimen described, which is a separate left valve, has a length of 

 28 nun., height 20 mm., and hinge-line IS mm. 



Formation and locality. In the shales of iUl' Chemung group at Philips- 

 burg, N. Y. 



Section e, arcoidea. 

 Leptodesma Maclvhii. 



PLATE XXV, KIGS. 8, M ; am> PLATE XCI, FIGS. 13, 14. 



Leptoiciiiia Madurii, Ham,. P;il. N. Y., voL v, pt. 1. Plate.s Jind Kxplanation-s : PI. 25, fig^. 8, 13. 



Jan., 1883. 



Shell large, i-huiiil)i)iil;il ; liody elongate, suh-elliptical, ol)li4ue, making an angle 

 of about 45° with the hinge-line ; length more tliaii one-thii'd greater than 

 the height ; aute-byssal margin oblique, genth' curving into a long, shallow, 

 undefined sinus ; ventral margin nearly straight, curving abruptly into tlic 

 posterior margin, wliidi is ronnilcil and ain uptly recurved. 



Left valve moderately convex in the lower part, becoming gibbous and 

 sub-angular along the line of the axis to the l)eak. Right valve distinctly 

 arcuate, less convex, narrowly gibbous in the umbonal region. 



Hinge-line straight; length about (Hjnal to the height of the valve. 



Beaks sub-anterior, directed forward, Imt little elevated above the hinge- 

 line in the left valve, depressed in the right. Umbonal angle acute. 



Anterior end narrow-triangular, limited by an oblique depression which 

 extends nearly one-third of the length of the valve to the byssal sinus. 



