LAMELLIBRANCHIA TA. 
39 
Aviculopecten tenuis. 
PLATE VII, FIGS. 27, 28; and PLATE LXXXI, FIG. 6. 
Aviculopecten tenuis, Hall. Pal. N. Y., vol. v, pt. 1. 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 90°. 
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. Byssal 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 
impression 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 min. 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, 
