LAM EL LIBRA NCHIA TA. 1 23 



Test thin, in the left valve marked by somewhat distant, elevated, lamellose, 

 concentric striae, with intermediate finer lines, and by abruptly elevated 

 rounded rays with wider interspaces; these arc cancellated by the concentric 

 stria?, which are more conspicuous on the wing. In the right valve the radi- 

 ating stria? are subdued, and the concentric stria' become more conspicuous. 



Interior unknown. 



A specimen of the left valve is 21 nun. in length, lb nun. in height, and 

 hinge-line 14 mm. 



This species differs from A. delta by its narrower form and greater obliquity 

 of body. 



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



N. Y. 



ACTINOPTERIA ZETA. 



PLATE XX111, FIG. 9; IND PLATE I.XX.YIV, FIGS. 1. .'. 



Aethinpteria ze.ta. Hall. Pal. N. Y., vol. v, jit. 1. Plates and Explanations: PI. "ill. fig !). Jan.. L883. 



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. (Jmbonal region 

 subtending nearly a right angle. 



Ear small, rounded at the extremity, defined by a strongly marked h\ ssal 

 depression. Wing large, joining the body below tin' middle, not strongly 

 defined; margin deeply concave in the middle ; extremity produced into an 

 acute extension. 



Surface marked by strong, elevated, regular, distant, lanielliform. concentric 



