LAMELLIBRANCHIATA. 235 



line. Umbonal region narrow and moderately gibbous, subtending a very 

 acute angle. 



The byssal depression reaches the mai'gin just below the beak, leaving a 

 very small, acute, anterior extremity. Wing large, undefined, joining the body 

 at the posterior end ; margin oblique, essentially straight, not recurved at the 

 cardinal extremity. 



Test thin, leaving upon the cast the marks of the stride of growth which 

 are more or less fasciculate upon the body, and crowded and lamellose in 

 front. 



Hinge furnished with a distinct posterior tooth. Interior unknown. 



The specimen described has a length of 32 mm., height 24 mm., and 

 hinge-line 22 mm. 



This species somewhat resembles L. Orodes, especially as shown in PI. xxv, 

 fig. 10, but the beak is more nearly anterior, the anterior end much smaller 

 and more acute, the byssal sinus not extending so low on the body of the 

 valve, and the body of the shell more distinctly attenuate toward the beak. 

 It is especially distinguished by its narrow slender beak and small anterior 

 end. 



Formation and locality. In a sandstone of the Upper Chemung group at 

 Warren, Pa. 



Leptodesma mytiliforme. 



PLATE xxv, FIGS. 7, 11 ; AND PLATE X(JI, FIGS. -22-35. 



Leptodesma mytUiformf, Hall. Pal. N. Y., vol. \', pt. 1. Plates and E.\planations : PI. 25, fig-s. 7, 11. 



Jan., 1883. 



Shell of medium size, elongate-ovate ; body narrowly elliptical, becoming 

 attenuate in front, oblique, making an angle of nearly 45° with the hinge- 

 line ; length more than one-third greater than the height ; ante-byssal margin 

 oblique, gently curving into a scarcely perceptible sinus, giving a straight or 

 slightly concave margin extending for two-thirds the length of the valve ; 

 posterior margin abruptly rounded. 



