348 Thirty-fifth Eeport on the State Museum. 



posteriorly and continuing in a direct line to the beak. Left valve 

 moderately convex. Eight valve unknown. Hinge-line straight, 

 greatly extended posteriorly, reaching beyond the margin of the 

 valve. Beak obtuse, prominent, nearly erect, situated on the 

 anterior third of the hinge. Umbonal region prominent, sloping 

 abruptly on the posterior, not well-defined on the anterior side. 

 Anterior end limited by a shallow, obscure sulcus ; margin regularly 

 rounded. Wing narrow-triangular, much extended along the 

 hinge, terminating in a mucronate process ; margin deeply and 

 acutely sinuate. 



Test thin, marked by fine, closely arranged, concentric striae, 

 which are more crowded anteriorly ; these are crossed by extremely 

 fine radii, which are chiefly confined to the shallow sulcus, anterior 

 to the beak, and are also obscurely marked on the wing. Liga- 

 mental area marked by two strong grooves. Two oblique linear 

 depressions along the posterior slope indicate the probable existence 

 of lateral teeth. Other characters of the interior unknown. 



This specimen has a length of 11 mm., height 7 mm., hinge-line 

 12 mm. 



This species bears some resemblance to several species in the 

 Chemung group, but it is comparatively narrower and more oblique. 



Formation and locality. In the Marcellus shale, Bloomfield, Ontario 

 county, N. Y. 



Leptodesma Eogersi. 



Leptodesma Rogersi, Hall. Pal. N.Y., vol. v, pt. 1. Plates and Explanations ; 

 • PI. 21, figs. 1-9. Jan., 1883. 



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

 oblique ; length greater than the height ; anterior and basal margins 

 broadly rounded ; posterior margin extended and abruptly recurved. 

 Valves equally convex above. Eight valve somewhat depressed be- 

 low, comparatively higher than the left. Hinge-line straight, longer 

 than the length of the shell. Beak sub-anterior, obtuse, nearly 

 erect, prominent. Umbonal region gibbous, oblique. The anterior 

 extremity is scarcely alate or auriculate, consisting of a rounded 

 extension, straight above and slightly sinuate at the base. Wing 

 comparatively large, triangular, joining the body of the valve near 

 the posterior extremity, defined by the crowding and curving of the 

 concentric striae ; margin nearly straight for five-sixths of its extent, 

 then acutely recurving; extremity prolonged into a mucronate spine 

 which extends beyond the posterior limit of the valve. In the right 

 valve the wing is less deeply sinuate. 



