182 PALAEONTOLOGY OF NEW YORK. 



more numerous and fasciculate at irregular intervals, producing a slightly 

 undulated surface, crowded and lamellose on the anterior. 



Interior unknown. 



A specimen of a lefl valve has a length, from beak to base, of 43 mm., 

 heighl "J'l mm., and binge-line to the base of the spine '!') mm. A right 

 \al\e. referred to this species, lias a length of 32 mm., height 27 mm., and 

 hinge-line, exclusive of the spine, 31 mm. 



This species is larger and more robust than L. longispinum ; the body is wider, 

 less oblique, and less abruptly recurving at the post-basal extremity, the wing 

 is larger and the greatest depth of the sinns not quite so near the cardinal 

 line: the striae do not recurve so abruptly forward on the posterior slope; the 

 right valve is proportionally higher ; the wing is less deeply sinuate, and the 

 spiniform extension is shorter. 



Compared with L. Mortoni,the body of the shell is less oblique and more 

 expanded, while the wing is much smaller and the sinus nearer the hinge-line. 



Formation and locality. In the middle portion of the Chemung group at 

 Painted Post, Steuben county. N. Y. 



Leptodesma Agassizi, n. sp. 



PLATE I. XXXIX. FIGS. 17-19 



Shell of medium size, sub-rhomboidal ; body broadly ovate, oblique to the 

 hinge-line at an angle of from ~>0- to ">5°; length nearly one-third greater than 

 tin' height : anterior margin broadly rounded below the byssal sinus; ventral 

 margin regularly curved; more abruptly curved at the post-basal extremity. 



Left valve moderate^ convex below, more convex above and gibbous on 

 the umbo. Right valve less convex. 



Binge-line straight, about equal to the height of the valve. 



Beaks sub-anterior, scarcely directed forward, prominent, rising a little 

 above the hinge-line. Umbonal region moderately gibbous, subtending an 

 acute angle. 



Anterior end Large, obtuse, truncate above and curving below into the 



