36 
PALEONTOLOGY OF NEW YORK. 
Beaks obtuse-angled, directed forward, placed anterior to the middle of 
the valve; sides of the umbo carinate, and descending abruptly to the ears. 
Posterior ear flat or concave, narrow-triangular, well-defined by the 
carinate sides of the umbo, and by the absence of the strong surface radii; 
margin concave; extremity acute, mucronate. Anterior ear about two-thirds 
as long as the posterior one, triangular, convex; defined by a deep sulcus, 
and the angular cardinal slope of the umbo; margins of the ears in the 
left valve straight or concave; in the right valve, convex; extremities acute- 
angled. In the left valve the byssal sinus is broad and rounded; in the 
right valve it is a deep, angular notch. 
Test of the left valve marked by from 40 to 50 regular, continuous, thread¬ 
like rays, alternating in fours, with three degrees of prominence, crossed and 
crenulated by fine, regular, concentric striae. The surface of the right valve 
is nearly the reverse of this, having broad, flattened rays, arranged in pairs, 
regularly bifurcating, with narrow concave interspaces which correspond to 
the rays of the opposite valve. The posterior ears show several delicate rays 
extending from the apex of the beak over their upper portion. 
The interior is not known. 
A specimen preserving both valves has a height of 14 mm., length 19 mm., 
and hinge-line 20 mm. A right valve has a height of 14 mm., length 22 mm. 
A left valve has a height of 12 mm., length 16 mm., hinge-line 17 mm. 
This species in appearance resembles A. ornatus, and differs in its longitu¬ 
dinally narrower outline, more obtuse beak, the numerous rays, and the 
absence of strong, lamellose, concentric fimbriae. The two forms are of a 
group resembling several species in a parallel group of the genus Actinoptera, 
which are remarkable as possessing an aviculoid character (when compared with 
some recent species of Avicula ); in the strong rays, the deep anterior sulcus 
and byssal sinus separating the anterior ears from the body of the shell. 
Formation and localities. In the soft shales of the Hamilton group at Tinker’s 
Falls, Onondaga county; Bellona, Yates county; and near Norton’s Landing, 
Cayuga lake, N. Y. 
