16 PALAEONTOLOGY OF NEW YORK. 



depressed convex; the anterior margin more convex, and the base abruptly 

 rounded. 



Valves convex, very similar, except that the right valve shows an acute 

 byssal sinus. 



Hinge-line short, straight, length more than one-half the length of the 

 shell. 



Beaks acute, prominent, anterior to the middle of the hinge. 



Ears small, broad-triangular ; posterior one a little the larger, margin 

 slightly concave ; anterior ear defined by a distinct sulcus, which is narrow 

 and deep in the right valve, margin convex ; byssal sinus deep and acute. 

 The posterior ear is less distinctly limited in the right valve. 



Test near the beaks ornamented by fine filiform rays, which are crossed 

 and crenulated by sharp, concentric strife. Below the middle of the shell 

 the rays become large rounded plications, increasing by implantation or 

 bifurcation, and the regular concentric stria? are stronger, distinctly crenu- 

 lating the rays, becoming foliate, and giving the surface a rugose appearance. 

 The rays and concentric lines are present on the ears. 



The interior is not preserved. 



Two left valves have a height of 35 mm., and 22 mm.; length 29 mm. and 

 18 mm. ; the hinge-line 16 mm. and 10 mm. respectively. A right valve 

 of nearly the same proportions has a height of 33 mm., length 25 mm., 

 and hinge-line 17 mm. 



This species is distinguished from A. duplicatus by its comparatively. greater 

 height, the obliquity of the valve, and the acute beaks ; the surface characters 

 of the two are very similar. A. cancellatus is more circular in outline, and the 

 anterior ears are larger than the posterior pair. 



Formation and localities. Shales of the middle Chemung group, Pliilipsburg, 

 Rockville and Hobbieville, Alleghany county, N. Y. 



