LAMELLIBRANCHIA TA . 
13 
were indicated has demonstrated the identity of the several forms, notably 
figs. 14 and 15 of plate 5, which do not show a fasciculation of the striae. 
Formation and localities. In the shales of the Hamilton group, Schoharie, 
Onondaga and Madison counties; also occurring in boulders of the coarse 
shales of the Hamilton group at Chemung Narrows, N. Y. 
Aviculopecten Idas. 
PLATE III, FIGS. 1, 2; PLATE XXIV, FIG. 4. 
Aviculopecten Idas, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 3, figs. 1, 2 ; 24, fig. 4. 
Jan., 1883. 
Shell ovate-orbicular, transverse axis slightly oblique to the hinge-line ; height 
nearly equal to or greater than the length; outline full in front, not extended 
posteriorly. 
Right valve depressed; moderately convex. Left valve not observed. 
Hinge-line straight, length less than two-thirds the length of the shell, 
extending farther towards the anterior than the posterior margin. 
Beaks obtuse, well-defined, the umbo separated on each side from the 
ears by a distinct sulcus, which is more marked on the anterior side. The 
beaks are nearly in the centre of the hinge-line, but somewhat anterior to 
the middle of the shell. 
Ears unequal. Anterior one about twice the size of the posterior, convex, 
margins convex; separated from the body of the shell by a deep sinus which 
is continued to the beak as a distinct sulcus. Posterior ear triangular, with 
margins concave. 
Test thin, marked by fine, concentric lines of growth, and about 80 
rounded, radiating striae, which increase by interstitial addition, with the 
interspaces less than the striae, of which there are from 7 to 9 in the space 
of 5 mm. at the pallial margin. The surface characters continue on the 
ears; but upon the posterior ear the radiating lines are very subdued, while 
on the anterior one they are quite prominent. 
Interior characters not observed. 
The largest specimen has a height and length nearly equal, and hinge-line 
