Lamellibbanchiata. 219 



This is one of the largest and most abundant species in the lower and 

 middle portions of the Hamilton group. It is very easily distinguished 

 by the large ears with the furrowed ligamental area, full, obliquely 

 ovate outlines, and characters of ornamentation. 



Formations and localities. In limestone referred to the age of the 

 Upper Helderberg group (Corniferous limestone), at Sandusky, Dela- 

 ware, and near Columbus, Ohio, and also at the falls of the Ohio ; 

 in the coarser shales of the Hamilton group, at numerous localities 

 in the eastern and central portions of the State ; in the upper soft 

 calcareous shales of the group at Moscow, Livingston county, N. Y., 

 and very rarely in the western extension of the group . 



AVICULOPECTEN PECTENIFOBMIS. 



Avicula pecteniformis, Conkad. Jour. Acad. Nat. Sci. Phila., vol. 8, p. 240. 



1842. 

 Aviculopecten pecteniformis (Conrad), Hall. Pal. N. It., vol. v, pt. 1. Plates 



and Explanations ; PI. 1, fig. 9. Jan., 1883. 



Shell sub-ovate oblique, to the hinge-line ; length equal to the height; 

 anterior and basal margins regularly rounded, the body of the shell 

 extending in a nearly straight line from the postero-lateral margin 

 to the beak. Left valve convex, reaching its greatest convexity 

 at about one-third the length of the valve from the beak. Right 

 valve not observed. Hinge-line straight, having a length of more 

 than two-thirds thB length of the shell. Umbo prominent, sloping 

 rapidly to the ears; the sides subtending a little more than a right 

 angle. Beak prominent, situated a little anterior to the middle 

 of the hinge. Ears triangular, unequal, the anterior about one-half 

 the size of the posterior one, lateral margins concave. 



Test ornamented by about 35 sharp, strong rays, with intermediate 

 finer ones, which are crossed by fine imbricating stride of growth. 

 The radii are not present on the ears. 



The original of this species has a length of 47 mm. with an equal 

 height. The hinge-line measures 27 mm. 



This form may be distinguished from A. princeps by its smaller an- 

 terior ear, and less expansion at the base of the ears. The umbo 

 appears more contracted, and in the single known specimen is more 

 prominent. The principal radii are sharper and stronger, and the 

 alteration of smaller ones more regular. 



In A. exacutus, which resembles this form, the beaks are nearer the 

 middle of the hinge-line, and the shell has a distinctly reticulate sur- 

 face ornamentation, with the radii continued over the ears. 



Formation and locality. From the Corniferous limestone of the 

 Upper Helderberg group, Schoharie, N". Y. 



