8 PALAEONTOLOGY OF NEW YORK. 



This well-defined species 16 readily distinguished by the slight obliquity of 

 the body of the shell to the hinge-line, and the marked characters of the 

 surface. It differs from A. princeps and A. pedeniformis in having strong and 

 regular concentric markings, while the rays in these species are much more 

 acute, and the axis of the shell is usually quite oblique to the hinge-line. 

 Specimens which preserve the exterior shell show very beautifully the short 

 spinous processes of the concentric lamellae ; as shown, of the natural size, in 

 fig. 10, pi. 3. Examples of the species are abundant, and have a considerable 

 vertical and horizontal range within the group in New York. 



Formation and localities. In the shales of the Hamilton group, at Pratt's falls, 

 and Delphi, Onondaga county; Ludlowville, Cayuga county; Bellona, Yates 

 county ; Geneseo, Livingston county, and other places in western New York. 



AviCULOPECTEN EXACUTUS. 



PLATE III, FIGS. 1&-22. 



Avieubipecten exacutus. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 3, tigs. 1S-22. 



Jan., 18S3. 



Shell of medium size, obliquely broad-ovate ; greatest longitudinal diameter 

 below the middle ; height nearly equal to the length, transverse axis oblique 

 to the hinge-line ; basal margin full and regularly rounded ; posterior margin 

 extended beyond the ear, and more convex than the anterior. 



Valves equally convex ; the byssal sinus larger in the right valve, and the 

 umbo less ample. 



Hinge-line straight, length four-fifths the longitudinal diameter, extending 

 nearly as far as the anterior margin. 



Beak obtuse, oblique, anterior to the middle of the hinge and of the valve ; 

 umbo ample. In some specimens of the left valve, the beak arches over the 

 hinge-line, while in the right valve the beak rises from the hinge. 



Ears triangular ; posterior one somewhat the larger, margin concave, 

 extremity acute ; defined from the umbo by a broad, shallow sulcus, an 

 obscure carination, and an abrupt change in the surface characters to sub- 



