Lamellibranchiata. 309 



actinopteria eta. 



Actinopteria eta, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell of medium size, rhomboidal ; body broad and short-ovate, 

 oblique at an angle of about 45° ; length about one-fourth greater 

 than the height ; anterior margin below the sinus nearly vertical, 

 curving gradually into the broad base; posterior side regularly 

 rounded. Left valve regularly convex below, moderately gibbous 

 above. Eight valve smaller, depressed-convex below, gently con- 

 vex above. Hinge-line straight, length greater than the height of 

 the shell. Beak in the left valve sub-anterior, prominent, arching 

 over the hinge ; in the right valve subdued, scarcely rising above 

 the hinge-line. Umbonal region gibbous in the left valve, subtend- 

 ing an acute angle. Ear in the left valve rounded at the extremity, 

 slightly oblique, denned by a distinct byssal depression. In the right 

 valve the ear is larger and flat, strongly limited. Wing compara- 

 tively large, extending three-fourths the length of the valve, not 

 strongly defined ; margin concave ; extremity produced and acute. 

 In the right valve the wing is larger, extending almost as far as the 

 posterior side of the valve. 



Surface of left valve marked by distant, lamellose, concentric striae 

 and by radiations with wider interspaces which have sometimes slen- 

 der intercalated rays. The concentric lamellae crenulate the radii, 

 and are arched backward in the interspaces, producing a beautifully 

 cancellated surface. The finer concentric striae between the lamellae 

 are rarely visible, and the spaces between the rays show, under a lens 

 and in well-preserved examples, extremely fine, crowded radiations. 

 The radii are also seen in the depression limiting the ear, and on 

 the wing they are equal to those on the body of the valve. The 

 right valve is marked by regular, equi-distant, concentric lamellae. 

 The posterior slope and wing show numerous fine radii which are 

 crossed by concentric striae continued from the lamellae on the body 

 of the valve. . ■ 



A small specimen is 20 mm. long, 14 mm. high, and hinge-line 

 16 mm. A larger imperfect example has, approximately, a length 

 of 30 mm., height 22 mm., and hinge-line 24 mm. 



In surface characters this species resembles A. zeta, but is dis- 

 tinguished by its greater obliquity, and much smaller wing. 



Formation and locality. In the lower part of the Chemung group 

 at Ithaca, N. Y. 



