LAMELLIBRANCHIA TA. 
179 
Leptodesma longtspinum. 
PLATE XXI, FIGS. 14, 17-19; AND PLATE LXXXIX, FIGS. 2-4. 
Avictda longispina, Hall. Geolog. Surv. N. Y.: Rep. Fourth Dist., p. 262, fig. 3. 1843. 
Pterinta longispina (Hall), S. A. Miller. Cat. Amer. Pal. Foss. 1877. 
Leptodesma longispinum. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 21, figs. 14,17-19. 
Jan., 1883. 
Shell above the medium size, sub-rhomboidal; body elongate-ovate, very 
oblique; length greater than the height, often nearly double; margin from 
the byssal sinus to beyond the base broadly rounded; posterior margin 
extended and abruptly recurved. 
Valves, in young specimens, sub-equally -convex; in older specimens the 
right valves are somewhat less convex. 
Hinge-line straight; length equalling or greater than the length of the valve. 
Beaks anterior, prominent, obtuse, nearly erect, arching over the hinge-line. 
Umbonal region gibbous, subtending an acute angle. 
Anterior extension scarcely auriculate, rounded in front, straight above. 
Wing small, very narrow-triangular; margin deeply sinuate, the bottom of 
the sinus being close beneath the hinge-line; extremity produced into an 
elongate spine: 
Test thin, marked by fine, irregular, concentric striae of growth, which 
are abruptly recurved on the post-cardinal slope, and extend along the margin 
for nearly half the length of the body before recurving into the wing. The 
striae are more elevated and crowded on the anterior side. 
The original specimen of this species has a greatest length from beak to 
base of 42 mm., height 24 mm., hinge-line about 38 mm. A small right 
valve has a length of 22 mm., height 12 mm., hinge-line 24 mm. 
This species resembles L. Rogersi, but the shell attains a larger size; the 
body is more oblique to the hinge-line, and the posterior extremity more 
abruptly recurved ; the wing is smaller and not extending so far down the body 
of the valve, the sinus is narrower with the deepest portion just beneath the 
hinge; the spine is stronger and more distinctly defined ; the right and left 
