Lamellibranchiata. 299 



valve, in old specimens, has the umbo convex above, depressed- 

 convex or flat below, while in young specimens the umbo is altogether 

 convex or gibbous. Eight valve moderately convex near the umbo, 

 flat or concave below the middle. Hinge-line straight, length about 

 one-fifth less than the length of the shell, not extending as far as 

 the posterior margin of the left valve, but extending beyond the 

 posterior margin of the right valve. Beak acute, directed forward, 

 somewhat prominent, anterior, nearly terminal. Umbonal region 

 (in young shells) well-marked by the sulci ; umbonal angle acute. 

 Ear very small, separated by a vertical sulcus. Wing large, trian- 

 gular, defined in young shells by the abrupt convexity of the post- 

 cardinal slope ; in old specimens not distinctly defined ; margin 

 concave ; extremity acute. The wing of the right valve is larger, 

 and extends beyond the margin of the shell. 



Test comparatively thin, marked with regular, sub-angular, alter- 

 nating radii, the stronger ones extending from the summit of the 

 umbo ; a second set intercalated on the lower part of the umbo, and 

 a third set of smaller rays coming in toward the margin. The first 

 and second series only are shown in young shells. In the right 

 valve the radii are more subdued. The rays extend over the cardinal 

 expansions of both valves in nearly uniform size. Concentric, 

 elevated, regularly rounded striae of growth cross the rays, and can- 

 cellate the surface, and at intervals are crowded together, giving it 

 a lamellose aspect. Ligamental area marked by one or two parallel 

 grooves. 



A large specimen has a length of 65 mm., height 47 mm,, hinge- 

 line 55 mm. A specimen of medium size has a length of 51 mm., 

 height 35 mm., hinge-line 41 mm. A smaller example has a length 

 of 18 mm., height 11 mm., hinge-line 13 mm. 



This species is similar in form to A. decussata, but differs in the 

 convexity of the valves and the less strongly decussated character of 

 the ornamentation, the rays being continuous and annulated by the 

 concentric striae, while in that species the rays are interrupted by 

 strong concentric lamellae, between which are the finer lines of growth. 

 This species is also less abundant and more restricted in geographical 

 range. 



Formation and locality. In the shales of the Hamilton group on 

 the shores of Oanandaigua lake, and near Bellona, Yates county, N. Y. 



