222 PALEONTOLOGY OF NEW YORK. 



A left valve has a Length of 51 nun., height 37 mm., binge-line about 

 12 mm. 



Specimens of this species usualh present a very rough and irregular aspect 

 from the character of the shell, with its strongly laniellose structure and the 

 coarse matrix in which it is usually found. The specimen figured is a cast of 

 tlie interior, and is distinguished by the broad posterior end and abrupt narrowing 

 toward the front, the obtusely sub-angular umbo, and the large anterior end. 

 In a crushed specimen retaining both valves, the right valve is almost equallj 

 convex with the left ; the surface is less lamellose and presents only gentle 

 undulations. 



Forma/ion and locality. In a coarse sandstone of the upper part of the 

 Chemung group, on the road from Olean, N. Y., to Smethport, Pa. 



Leptodesma Biton, n. sp. 



PL UK XCI, FIG I. 



Shell large, sub-rhomboidal ; body elongate-ovate, making an angle of about 



45° with the hinge-line; height nearly two-thirds of the length ; ante-bvssal 

 margin oblique, slightly curving into a long, shallow sinus, which impresses 

 the margin for more than half the length of the valve; basal and posterior 

 margins broadly rounded. 



Left valve regularly and gently convex in the lower part, becoming more 

 convex and gibbous above the middle. Right valve unknown. 



Hinge-line straight, greater than the height of the valve. 



Beak sub-anterior, directed forward, but little elevated above the hinge- 

 line. Umbonal region abrupt!) gibbous, subtending an acute angle. 



Anterior end produced, narrow, acute at the extremity, bruited by an 

 oblique, shallow byssal depression. Wing large, joining the body at the 

 posterior end. not distinctly separated; margin slightly oblique below, gently 

 concave above the middle, and curving slightly outward just below the car- 

 dinal line. 



Tesl thin: cast marked by line concentric stria' of growth, which, at 



