356 Thirty-fifth Befort ok the State Museum. 



about 22 mm. A detached right valve has a length of 29 mm., 

 height 20 mm., hinge-line 30 mm. 



This species resembles L. robustum in general expression and in the 

 sinus of the wing, but the body is narrower, more regularly convex, 

 the base less expanded, and the basal margin and the striae along the 

 posterior slope are not so abruptly recurved. 



Formation and localities. In the Chemung group, near Corning, 

 Chemung county, and Portville, Cattaraugus county, N". Y. 



Leptodesma disparile. 



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

 tions ; PI. 25, figs. 2-4. Jan., 1883. 



Shell of small or medium size, sub-rhomboidal ; body ovate, oblique 

 to the hinge at about 45°; length more than one-third greater than 

 the height in the left valve ; ante-byssal margin oblique, gently 

 curving into the sinus ; basal and posterior margins regularly 

 rounded. Left valve convex below, gibbous above. Eight valve 

 flat or concave below, depressed-convex in the middle, and gibbous 

 on the umbo. Hinge-line straight, greater than the length of the 

 shell. Beak sub-anterior, prominent, rising above the hinge-line, 

 directed slightly forward. Umbonal region gibbous, subtending an 

 acute angle. Anterior end large, somewhat produced, limited by a 

 nearly vertical byssal depression ; extremity obtusely angular. 

 Wing somewhat large, joining the body near the posterior extremity; 

 margin oblique below, concave above the middle, and produced into 

 an elongate spinif orm process. The right valve is smaller, the wing 

 more extended and less defined, while the anterior end is large and 

 well-defined by the byssal sinus and sulcus. 



Test marked by concentric, lamellose ridges with intermediate 

 finer lines of growth ; the lamellae are sometimes more distinct on 

 the right valve. 



A left valve has a length of 28 mm., height 20 mm., hinge-line 

 more than 30 mm. A right valve has a length of 30 mm., height 

 18 mm., hinge-line 33 mm. 



This species is distinguished from L, Rogersi and L. spinigerum by 

 the large anterior end, narrower umbo, more extended wing, less deeply 

 sinuate margin, less convex right valve, and the regular concentric) 

 lamellose ridges of the surface. 



