226 PALEONTOLOGY OF NEW YORK. 



I, .ft valve moderately convex in the middle, gibbous on the umbo, 

 depressed-convex in the lower part. Right valve unknown. 



Hinge-line straight ; length about equal to the height of the shell. 



Beak obtuse, directed Forward, moderately prominent, situated anterior to 

 the middle of the cardinal line. I 'mhonal region gibbous, subtending an 

 acute angle. 



Anterior end comparatively large, produced into a aasute extension. 

 Wing small, oarrow-triangular, joining the body of the shell more than one- 

 third the height above the base: margin gently concave; extremity acute. 



Test thin, marked by fine regular stride of growth, which are fasciculate 

 on the umbo and lamellose at nearly equal intervals on the lower part of 

 the shell. 



Interior unknown. The hinge is marked by a single slender groove. 



A left valve has a length of 40 mm., height 27 mm., hinge-line about 

 27 nun. 



This species resembles L. complanatum, but differs in its greater proportional 

 length and the body is more depressed and extended below. It is characterized 

 by the spatulate posterior extension, which is more extreme than in any other 

 species here described. 



Formation and locality. In the Chemung group, Lawrenceville, Tioga count v. 

 Pa. 



Leptodesma patultjm, n. sp. 



PL \TK XI l. PIGS I", n 



Shell above the medium size, sub-rhomboidal ; body oblique, elongate, sub- 

 ovate: length slightly more than one-third greater than the height; 

 ante-byssal margin oblique, gradually curving to the ventral margin which 

 is nearly straight : posterior margin broadly curved. 



Valves moderately convex below, scarcely gibbous above, similar in form: 

 the right valve being a little smaller and less convex than the left. 

 Hinge-line straight, somewhat longer than the height of the shell. 

 Beaks at about the anterior third of the hinge-line, directed forward. 



