230 
PALAEONTOLOGY OF NEW YORK. 
Hinge-line straight; length about equal to the height of the shell. 
Beaks prominent, directed forward and arching over the hinge, situated 
at about the anterior third of the cardinal line. Umbonal region angularly 
gibbous above, abruptly defined along the posterior slope. Umbonal angle 
acute. 
Anterior end short, angular. Wing an undefined, narrow-triangular 
expansion; margin straight or a little concave; extremity very slightly 
produced. 
Test moderately thick, marked by fine concentric strise, which are 
fasciculate on the body of the shell, and crowded and lamellose over the 
anterior side. 
Interior unknown. 
A large left valve has a length of 60 mm., height 36 mm., and hinge¬ 
line 37 mm. 
This species is distinguished from L. Lidias by its larger and stronger form, 
being very gibbous along the entire length of the shell, and with a propor¬ 
tionally smaller wing. 
Formation and locality. In the upper part of the Chemung group at Mansfield, 
Tioga county, Pa. 
Leptodesma Phaon, n. sp. 
PLATE XCI, FIG. 18. 
Shell above the medium size, narrow-rhomboidal; body sub-cylindrical, very 
oblique, making an angle with the hinge-line of less than 30°; length nearly 
twice the height; ante-byssal margin short, oblique, curving into the broad, 
almost straight ventral side; posterior margin abruptly but regularly rounded 
and continuous with the base of the wing. 
Left valve very convex, gibbous above. Right valve unknown. 
Hinge-line straight, somewhat longer than the height of the shell. 
Beak at the anterior third of the hinge-line, obtuse, moderately elevated, 
directed forward. Umbonal region gibbous, not well-defined anteriorly, 
abruptly depressed on the posterior. Umbonal angle acute. 
