LAMELLIBRANCHIATA. 13 



were indicated has demonstrated the identity of the several forms, notabl}* 

 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. i. 



Aviculopecteii Idas, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 3, tig-s. 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 ))y line, 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 



