3G PALEONTOLOGY OF NLW YORK. 



Hcaks (jl)tiist.'-aii;ik'il, (lirLTtril I'orwunl, placed aiitorior to the luiddlf of 

 tlu' valve; sides of the uinho carinate, and descending al)riiptly to the ears. 



Posterior ear flat or concave, narrow-triangular, wll-dcrnu'd liy the 

 carinate sides of the umi1)o. am! liy the absence of the strong surface radii ; 

 niarsrin concave; t-xtriMiiitN acute, iiuicronate. Anterior car alioiit two-thirds 

 as long as the posterior one, triangular, convex; defined tiy a deep sulcus, 

 ami the angular cai'dinal slope of the uud)o; margins of the ears in the 

 left valve straight or concave ; in the right valve, convex ; extremities acute- 

 angled. In the left valve the byssal sinus is liroad and rounded ; in the 

 right valve it is a deep, angidar notch. 



Test of the left valve marked by from iU to -"jU regular, continuous, thread- 

 like ray.s, alternating in fours, with three degrees of prominence, crossed and 

 crenulated by fine, regular, concentric striae. The surface of tiie rigiit valve 

 is neaily the reverse of this, having broad, flattened rays, arranged in pair.s, 

 regularly liifui'cating, with iiari-ow concave interspaces which coiii'spond to 

 the rays of the opposite valve. The posterior ears show several delicate rays 

 e.vtending from the apex of the beak over their upper portion. 



The interior is not known. 



A specimen preserving both valves has a height of 14 nnn., length 10 mm., 

 and hinge-line 20 nnn. A right valve lias a height of 1 1 nun., length 1*1' mm. 

 A left valve has a height of 12 nun., length IG mm., hinge-line 17 mm. 



This species in appearance resembles A. ornatvs, and differs in its longitu- 

 dinally narrower outline, more obtuse beak, the mmierons rays, and the 

 ab.sence of strong, lamellose, concentric fimljriav The two forms are of a 

 group resembling several species in a parallel group of the genus Actinopter.v, 

 whi(di are rennirkable as possessing an aviculoid character (when compared with 

 •some recent species of Avicula) ; in the strong rays, the deep anterior sulcus 

 and byssal sinus separating the anterior ears from the body of the shell. 



Formation and localities. In the .soft shales of the Hamilton group at Tinkler's 

 Falls, Onondaga county; Hcllona, Vates county; and near Norton's Landing, 

 Cayuga lake, N. Y. 



