8 PALAEONTOLOGY OF NEW YORK. 



Tlii.s \V(.'ll-(lc'liiR'il spccii's is roiidily distinguished by the slight obliquity of 

 the l)0(ly of the shell to tlic liinge-line, and the marked characters of the 

 surfacf. it (lill'cis iVoni A. princeps i\w\ A. pedeniformis in iiaving strong and 

 regular concentric markings, whih- the rays in these species are niucli more 

 acute, and tlie axis of the shell is usually <|iiite oi)rK{m' to the hinge-line. 

 Specimens wliicli preserve tlu' exterior shell show very beautifully the sliort 

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

 fig. 10, pi. ;i. E.xamples of the species are abundant, and have a considerable 

 vertical and horizontal range within the grouj] in New York. 



Formalim and localities. In the shales of tlie Hamilton grouj), at Pratt's falls, 

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

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



AVICDLOPECTEN EXACUTUS. 



PLATE lU, FIGS. 18-22. 



Aviculnpecten acaciUus, Hall. Pal. N. V., vol. v, i)t. 1. Plates and Explanations: PI. 3, tig-s. 18-22. 



Jan., 18S3. 



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

 l)elow the middle ; height nearly equal to the length, transverse axis oblique 

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

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



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

 umbo less ample. 



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

 nearly as far as the anterior maigin. 



Beak o])tuse, oblique, anterior to tlie miilille of tiic hinge and of the valve ; 

 undx) ample, in some specimens of the left \;dve, the lieak 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 broiwl, shallow sulcus, an 

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



