LAMELLIBRANCHIATA. 39 



AVICULOPECTEN TENUIS. 



PLATE VII, FIGS. .>7, 28; AND PLATE LXXXI, FIG. 0. 



Aviculopecten tenuis, Hall. P;tl. N. Y., vol. v, pt. ]. Plates and Explanations: PI. 7, figs. 27, 28. 



Jan., 1883. 



Shell small to medium size, sub-orbicular, transverse axis oblique ; length and 

 height equal ; margins regularly rounded, somewhat extended behind. 



Left valve slightly convex, nearly flat. Right valve unknown. 



Hinge-line straight, nearly central, ; length five-sixths of the longitudinal 

 diameter. 



Beak directed forward, not rising above nor extending beyond the hinge, 

 anterior to the centre of the hinge-line and of the valve. Umbonal angle 9(P. 



Ears large, broad-triangular, unequal, separated from the sides of the umbo 

 by a broad, shallow sulcus. Anterior ear about one-half the size of the 

 posterior, flat ; margin convex ; extremity obtuse-angular. By ssal sinus broad 

 and comparatively shallow. Posterior ear marked by a low, convex undula- 

 tion, adjacent and parallel to the sulcus ; margin straight or slightly concave; 

 extremity rectangular, pointed, more obtuse in mature specimens. 



Test marked by about twenty-five strong, regular radii, with broader 

 concave interspaces, and also, in well-preserved specimens, smaller interme- 

 diate rays. The specimens described are casts of the interior, and show but 

 faint traces of concentric lines of growth. The ears are mostly free from 

 surface markings. 



Pallial line impressed, continuous, situated about one-half the distance 

 between the apex of the beak and the pallial margin of the shell. Muscular 

 imjjression elliptical, within the concavity of the valves adjacent to the 

 sulcus of the posterior ear. Hinge-ligament without marked characters. 



The largest specimen has a height of 29 mm., length 30 mm., and hinge- 

 line about 23 mm. A smaller individual is 23 mm. in height, 23.5 mm. in 

 longitudinal diameter, and hinge-line measuring 19 mm. 



This species resembles in some respects A. ignotus, and is distinguished by 

 its transverse form, greater obliquity, larger anterior ear in the left valve. 



