388 Thirty-fifth Report on the State Museum. 



margin ; posterior margin abruptly rounded. Left valve depressed- 

 convex, a little gibbous on the umbo. Right valve unknown. 

 Hinge-line straight ; length less than the height of the shell. 

 Beak acute, incurved, directed forward, moderately prominent. 

 Umbonal region indistinctly denned anteriorly ; the posterior side 

 limited by the post-umbonal depression. Anterior end short; ex- 

 tremity 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 striae, which are 

 more crowded anteriorly, and very regular on the wing. 



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

 length of 28 mm., height 20 mm., and hinge-line 18 mm. 



Formation and locality. In the shales of the Chemung group at 

 Philipsburg, N. Y. 



Section e, arcoidea. 

 Leptodesma Maclurii. 



Leptodesma Maclurii, Hall, Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 25, figs. 8, 13. Jan .,1883. 



Shell large, rhomboidal ; body elongate, sub-elliptical, oblique, mak- 

 ing an angle of about 45° with the hinge-line ; length more than 

 one-third greater than the height ; ante-byssal margin oblique, 

 gently curving into a long, shallow, undefined sinus ; ventral mar- 

 gin 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, i 

 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. Wing large, undefined ; margin very oblique in the 

 lower part, slightly concave above, the greatest concavity being a 

 little below the hinge-margin ; extremity produced into an angular 

 extension. The right valve shows less distinction between the body 

 of the valve and the wing. 



Test thin, marked by distant lamellose lines of growth with 

 intermediate finer striae. These lines are more regular upon the 



