12 
PALEONTOLOGY OF NEW YORK. 
convex towards the middle of the postero-lateral side, and extending in a 
straight or slightly concave line to the beaks. 
Yalves moderately convex, the right one more depressed when seen in 
connection with the other, shorter and comparatively less oblique. 
Hinge-line straight, length nearly two-thirds the height of the shell; 
extending, in young individuals, as far as the anterior margins. 
Beaks prominent, well-defined, situated anterior to the middle of the 
hinge-line. Umbo very convex. 
Ears of the left valve triangular, the posterior one larger, more attenuate 
at the extremity, and defined by a sulcus; the margins concave from the 
rapidly sloping side of the prominent umbo. Ears of the right valve much 
narrower, and attenuate at the extremities, with fewer radiating lines, 
marked by a very deep byssal sinus. 
Test ornamented with numerous filiform, radiating striae, which are often 
fasciculate, and fine concentric lines of growth. The broad fascicles of striae 
often extend across one interval of growth, and on the succeeding interval 
appear as regular or irregular striae without aggregation, or as fascicles of 
finer striae. This change does not always occur, but is very characteristic 
when seen, as in figure 17, plate 5, where the variation is so extreme that it 
affects the outline of the shell. In young specimens the rays are regular, 
with a slight fasciculate arrangement. The concentric lines are sharper and 
more crowded on the ears and the rays are more subdued. 
Interior not observed. 
One specimen has a height of 30 mm., and a length of 32 mm.; another 
specimen has a height of 23 mm., and a length of 25 mm.; the hinge-line 
is 16 mm. 
The form and surface markings distinguish this species. It differs from A. 
formio, A. princeps and A. pedeniformis in its surface markings; and from A. 
princeps especially, by the presence of radii on the wings. 
Several varieties were designated in the preliminary examination as A. 
repletus and A. Orestes; but a larger collection of specimens since these species 
