36 PALAEONTOLOGY OF NEW YORK. 



Beaks obtuse-angled, directed forward, placed anterior to the middle of 

 the valve; sides of the umbo carinate, and descending abruptly to the ears. 



Posterior ear Hat or concave, narrow-triangular, well-defined by the 

 carinate sides of the umbo, and by the absence of the strong surface radii ; 

 margin concave ; extremity acute, mucronate. Anterior ear about two-thirds 

 as long as the posterior one, triangular, convex: defined by a deep sulcus. 

 and the angular cardinal slope of the umbo ; 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 broad and rounded: in the 

 right valve it is a deep, angular notch. 



Test of the left valve marked by from 40 to 50 regular, continuous, thread- 

 like rays, alternating in fours, with three degrees of prominence, crossed and 

 crenulated by fine, regular, concentric striae. The surface of the right valve 

 is nearly the reverse of this, having broad, flattened rays, arranged in pairs, 

 regularly bifurcating, with narrow concave interspaces which correspond to 

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

 extending 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 mm., length 19 mm., 

 and hinge-line 20 mm. A right valve has a height of 14 mm., length '22 mm. 

 A left valve has a height of 12 mm., length 16 mm., hinge-line 17 mm. 



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

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

 absence of strong, lamellose, concentric fimbria 1 . The two forms are of a 

 group resembling several species in a parallel group of the genus ACTINOPTERA, 

 which arc remarkable 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 Tinker's 

 Kails, Onondaga county ; Bellona, Yates county ; and near Norton's Landing, 

 < !aj uga lake, X. Y. 



