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 left valve has a length, from beak to base, of 43 mm., 
height 29 mm., and hinge-line to the base of the spine 25 mm. A right 
valve, referred to this species, has 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 sinus not quite so near the cardinal 
line; the strise 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 LXXXIX, FIGS. 17-19. 
Shell of medium size, sub-rhomboidal; body broadly ovate, oblique to the 
hinge-line at an angle of from 50° to 55°; length nearly one-third greater than 
the height; anterior margin broadly rounded below the byssal sinus; ventral 
margin regularly curved; more abruptly curved at the post-basal extremity. 
Left valve moderately convex below, more convex above and gibbous on 
the umbo. Right valve less convex. 
Hinge-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 
