LAMELLIBRANCHIA TA. 
227 
somewhat prominent. Umbonal region moderately gibbous, not strongly 
defined. Umbonal angle acute. 
Anterior end large, produced into a nasute extension. Wing large, 
joining the shell at the posterior extremity; margin scarcely concave, 
abruptly produced along the hinge-line. 
Test marked by fine lines of growth which are irregularly fasciculate, 
sometimes appearing as equidistant, sharply elevated lamellae; crowded on 
the anterior; more regular over the wing and abruptly recurving just 
beneath the hinge. 
The hinge is marked by one or two narrow, parallel, longitudinal grooves. 
A specimen of medium size has a length of 40 mm., height 26 mm., and 
hinge-line 28 mm. A similar right valve has a length of 36 mm., height 
24 mm., and hinge-line 27 mm. 
This species differs from L. jiaccidum in its much larger wing, which is more 
extended along the body of the shell. It is less convex and the umbo less 
gibbous than in any of the preceding species. 
Formation and locality. In the upper part of the Chemung group at Mans¬ 
field, Tioga county, Pa. 
Leptodesma complanatum. 
PLATE XXII, PIG. 2. 
Leptodesma complanatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 22, fief. 2. 
Jan., 1883. 
Shell of medium size, obliquely semi-elliptical; body sub-ovate, moderately 
oblique; length one-third greater than the height; ante-byssal margin sub¬ 
truncate, gradually curving into the broad ventral margin ; posterior margin 
abruptly rounded. 
Left valve depressed-convex, a little gibbous on the umbo. Right valve 
unknown. 
Hinge-line straight; length less than the height of the shell. 
Beak acute, incurved, directed forward, moderately prominent. Umbonal 
