Lamellibranchiata. 221 



left valve is one-half the width of the posterior, but of nearly equal 

 length ; in the right, valve the ear is larger, and has a well-marked 

 byssal sinus. The right valve is somewhat less convex than the left, 

 but, with the exception of the deeper anterior sinus, is very similar 

 in character. 



Test ornamented by about sixty strong, rounded rays, with some 

 smaller intermediate elevated lines, which are crossed by regular, 

 prominent, subimbricating lamellae of growth about 1.5 mm. apart. 

 On crossing the rays, the varices of growth are bent suddenly up- 

 ward and backward, forming short semi-tubular spines. The strong 

 rays are absent upon the ears, but the lines of growth are numerous, 

 and sharper than over the body of the shell. Numerous individuals 

 have a height of 55 mm., with usually a slightly greater length. 

 The length of the hinge-line in mature examples is about 45 mm. 



This well-defined species is 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. pecteni- 

 formis 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. 



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

 Pratt's jfalls, and Delphi, Onondaga county; Ludlowville, Cayuga 

 county ; Bellona, Yates county ; Geneseo, Livingston county, and 

 cither places in western New York. 



AVICULOPECTE^" EXACUTUS. 



Aviculopecten exacutus, Hall. Pal. N. T., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 3, figs. 18-22. Jan., 1883. 



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

 diameter below the middle ; height nearly equal to the length, trans- 

 verse axis oblique to the hinge-line ; basal margin full and regularly 

 rounded ; posterior margin extended beyond the ear, and more con- 

 vex 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, tke 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, 



