If, PALMONTOLOGY OF NEW YORK. 



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

 rimnded. 



^'alv(•s coin (-'X, very similar, except that the right valve shows an acute 

 hyssal sinus. 



Hiii-ro-line sliort, 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 unc 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 

 anil crenulated by sharp, concentric stria). Below the middle of the shell 

 the rays become large rounded plications, increasing ])y implantation or 

 bifurcation, and the regular concentric stride are stronger, distinctly crenu- 

 lating the I'ays, becoming foliate, and giving the surface a rugose appearance. 

 The rays and concentric lines arc present on the ears. 



The interior is not preserved. 



Two left valves have a height of 35 nun., and 'I'l 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 nini. 



This species is distinguished from A. duplicatus by its comparativeh' greater 

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

 of tiie 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, Philipsburg, 

 Itockville and Hobl)ieville, Alleghany county, N. Y. 



