Lamellibranchiata. 225 



Formation and localities. In the shales of the Hamilton group, 

 Schoharie, Onondaga and Madison counties ; also occurring in bould- 

 ers of the coarse shales of the Hamilton group at Chemung Narrows, 

 N. Y. 



AviCULOPECTEN IDAS. 



Aviculopecten Idas, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations; 

 PI. 3, figs. 1,2; pi. 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. Eight valve depressed; moder- 

 ately convex. Left valve not observed. Hinge-line straight, length 

 less than two-thirds the length of the shell, extending farther 

 toward 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 center 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 char- 

 acters continue on the ears ; but upon the posterior ear the radiating 

 lines are very subdued, while on the anterior one they are quite 

 prominent. 



The largest specimen has a height and length nearly equal, and 

 hinge-line of 20 mm. Another individual has a height and length 

 of 27 mm., and hinge-line of 16 mm. 



This species differs from A. fasciculatus in its comparatively larger 

 anterior and smaller posterior ears, while the shell is less full behind, 

 and the fasciculate surface has not been observed. In A. formio the 

 posterior ears are much larger, and the surface is distinctly reticulate. 

 A, scabridus the beaks are more rounded, the wings more extended, 

 and the surface quite different. Some forms of A. duplicatus and A. 

 rugasfriatus resemble this species; but the posterior ears of those species 

 are larger, the anterior pair wider, and the surface marking charac- 

 teristic. 



A careful comparison with A. Itys reveals the following differences : 

 A. Itys is less oblique in outline, radiating strias less distinctly rounded, 

 [Sen. Doc. No. 38.] 29 



