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. 



Valves moderately convex, the right one more depressed when seen in 

 connection with the other, shorter anil 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 snlcns ; 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 stria 1 , which are often 

 fasciculate, and fine concentric lines of growth. The broad fascicles of stria' 

 often extend across one interval of growth, and on the succeeding interval 

 appear as regular or irregular stria? 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. pecteniformis 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 



