198 PALEONTOLOGY OF NEW YORK. 



Test thin, marked by concentric stria', which, on the body of the shell, 

 arc crowded into fascicles and assume a distinct regularity in passing over 

 the wing. The binge shows one or two slender parallel grooves. 



Three similar specimens of the left have respectively the following 

 dimensions: length -VI, 32 and 33 mm., height 'I'l. '_'•"> and 23 mm., and 

 hinge-line 25, -!7 and 25 mm. 



In this species the body of the shell resemhles L. rohustum and L. potens, but 

 is less oblique to the hinge-line, the wing less defined, and its posterior 

 extremity not produced into a spine. 



Formation and locality. In the upper part of the Chemung group, Lawrence- 

 ville. Tioga county. Pa. 



Leptodesma umbonatum. 



PLATE XXII, FIG. 13; and PLATE XI. Hi. 9 



/. ;■ nbonatum, Hall. Pal. N. V., vol. v. pt. 1. Plates ami Explanations : PI. l'2. fig. 13. 



.Ian., 1883. 



Shell of medium size, rhomboidal : body oblique, narrowly ovate ; greatest length 

 one-half more than the height ; ante-byssal margin straight or slightly 

 convex, curving into the wide sinus, oblique below; ventral margin broadly 

 rounded ; posterior extremity moderately produced, abruptly recurved. 



Left valve convex, ventrieose above the lower third. Right valve 

 unknown. 



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



Beak at about the anterior third of the cardinal line, directed slightly 

 forward, prominent and arching over the hinge. (Jmbonal region narrow, 

 and ventrieose. defined anteriorly by a broad, shallow depression, which is 

 neaii\ vertical to the hinge-line, and posteriorly by the abrupt depression of 

 the body. 



Anterior end acute, triangular. Wing rather large, triangular, convex, 

 joining the body of the shell near the posterior extremity; margin broadly 

 concave; extremity produced, acute. 



