228 PALAEONTOLOGY 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 stria 1 , 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 

 2S mm., height 20 nun., and hinge-line 18 mm. 



Formation and locality, [n the shales of the Chemung group at Philips- 

 burg, N. Y. 



Section e, arcoidea. 

 Leptodesma Maclurii. 



PLATE SXV, FIGS 8, IS; ami PLATE XCI, FIGS. 13, 14. 



Leptodesma Maclurii, Hall. Pal. N. V.. vol. v. pt. 1. Plate-; ami Explanations: PI. 25, tigs. S, 13. 



Jan., lss:i. 



Shell large, rhomboidal ; body elongate, sub-elliptical, oblique, making an angle 

 of about 45° witli the hinge-line ; length more than one-third greater than 

 the height: ante-byssal margin oblique, gently curving into a long, shallow, 

 undefined sinns : ventral margin nearly straight, curving abruptly into the 

 posterior margin, which is rounded and abruptly recurved. 



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

 sub-angular along the line of the axis to the beak. Right valve distinctly 

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



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



Beaks sub-anterior, directed forward, but 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. 



