308 Thirty-fifth Report osr the State Museum. 



ACTIN-OPTERIA ZETA. 



Actinopteria zeta, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations ; 

 PI. 23, fig. 9. Jan., 1883. 



Shell large, rhomboid-orbicular ; body broadly ovate, nearly erect ; 

 height greater than the length ; auricular margin rounded into the 

 byssal sinus ; below the sinus the margin is nearly vertical and 

 gradually joining the broad basal curvature ; posterior end regularly 

 rounded. Left valve gently convex below, moderately gibbous 

 above. Right valve flat or concave below, depressed-convex above. 

 Hinge-line straight, much longer than the greatest length of the 

 valve. Beak in the left valve sub-anterior, elevated, curving over 

 the hinge ; in the right valve depressed, not rising above the hinge- 

 line. Umbonal region subtending nearly a right angle. Ear small, 

 rounded at the extremity, defined by a strongly marked byssal de- 

 pression. Wing large, joining the body below the middle, not 

 strongly defined ; margin deeply concave in the middle ; extremity 

 produced into an acute extension. 



Surface marked by strong, elevated, regular, distant, lamelliform, 

 concentric striaa, with intermediate finer lines, and by slender, fili- 

 form, undulating, distant rays, which are more crowded on the 

 anterior side, becoming finer and more numerous at the junction of 

 the wing with the body, and a few much stronger ones along the 

 cardinal line. Rays not present on the ear, which is marked only 

 by the crowded concentric striae. The concentric striee crenulate 

 the rays and bend backward in the interspaces, producing a beauti- 

 fully cancellated surface. The right valve shows two oblique lateral 

 folds or teeth posterior to the beak. 



A right valve has a length of 25 mm. parallel to the hinge, height 

 28 mm., hinge-line about 34 mm. An imperfect left valve has 

 an approximate length of 22 mm., height 24 mm., and hinge-line 

 33 mm. 



This species is distinguished by its erect, orbicular form, great ex- 

 tension of hinge-line, and marked surface characters. 



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

 at Ithaca, 1ST. Y. 



