LAMELLIBRANCMA TA. 
177 
Test thin, marked by closely arranged concentric striae, which at irregular 
intervals are crowded into fascicles, producing a gently undulated surface. 
On the wing the striae are closely arranged, and just below the hinge-line 
are turned backward along the spiniform extension of the wing. 
Interior unknown. Ligamental area narrow, having several fine grooves 
parallel to the hinge. 
One specimen has a length from beak to base of 24 mm., height 15 mm., 
hinge-line about 20 mm. The corresponding right valve has a length of 
20 mm., height 15 mm., hinge-line 26 mm. The largest specimen observed 
has a length of 32 mm., height 21 mm., hinge-line, to the origin of the 
spine, 20 mm. Small specimens are often less than 10 mm. in length and 
height. 
This species, occurring abundantly in the Hamilton group, closely resembles 
Avicula spinigera of Conrad, from the Chemung group. In the comparison of 
large numbers of specimens, the differences are so slight that specific distinc¬ 
tion is not always apparent. The Chemung forms are usually more oblique, 
but this is not a constant character; the wing is smaller and less extended 
along the posterior slope; the right valve is narrower and more oblique, and 
the sinus in the margin of the wing is narrower and deeper than in the right 
valve of the Hamilton form. 
Formation and localities. In the shales of the Hamilton group at Norwich, 
Chenango county; Middleville, Schoharie county, and Leonardsville, Madison 
county, N. Y. 
Leptodesma spinigerum. 
PLATE XXI, FIGS. 10-13; AND PLATE LXXXIX, FIG. 1. 
Avicula spinigera, Conrad. Jour. Acad. Nat. Sci., Phila., vol. 8, p. 287, pi. 12, fig. 3. 1842. 
Pteronites spinigerus (Conrad), S. A. Miller. Cat. Amer. Pal Foss., p. 202. 1877. 
Leptodesma spinigerum (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 21, figs. 
10-13. Jan., 1883. 
Shell of medium size, sub-rhomboidal; body ovate, oblique to the hinge at an 
angle of about 45°; length one-third greater than the height; ante-byssal 
23 
