12 PALJEONTOLOGY OF NEW YORK. 



convex towards tin- inidillo tjf the postero-lutcnd .side, and oxtendiiig in a 

 strai^rlit or slijrlitly concave line to the beaks. 



N'alves moderately convex, the right one nicjre depressed when seen in 

 connection with the otlicr, shorter and comparatively less ol)Ii(iue. 



Hinge-line straight, length nearly two-thinls tlic lu'igiit i>t' the shell; 

 extending, in yonng individuals, as far as the antiiim- iiinrgiiis. 



Beaks prominent, well-defined, situated anteiior 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 hy 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, 'flic hroad fascicles of striae 

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

 appear as regular or ii-rcgular striae without aggregation, or as fascicles of 

 finer stria?. This change does not always occur, hut 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 raj-s 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 l(iii:tli oi' '.VI nun. ; another 

 specimen has a height of 23 nun., and a length of 2") mm.; the hinge-line 

 is IG nnn. 



The form and surface markings distinguish this species. Tt diflers from ^1. 

 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. 

 replelus and A. Orestes ; hut a larger collection of specimens since these species 



