LAMELLIBRANCHIATA. 123 



Test thill, ill the left valve marked by somewhat distant, elevated, lamellose, 

 concentric strife, with intermediate finer lines, and by aln'uj)tly elevated 

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

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

 ating striae are subdued, and the concentric stria> become more conspicuous. 



Interior unknown. 



A specimen of the left valve is 21 mm. in length, 16 mm. in height, and 

 hinge-line 14 mm. 



This species differs from A. delta b\- 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 XXIII, FIG. ;i ; and PLATE LXXXIV, FIGS. 1, i. 



Acthwptei'ia zeta. Hall. Pal. N. Y., vol. v, pt. 1. Plates ami Explanations : PI. 23. tifr 3. .Tan., 1SS8. 



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

 tliau tlie length ; auricular margin rounded into the byssal sinus ; l)elow the 

 sinus the margin is nearly vertical and gradually joining the broad basal 

 curvature ; posterior end regularly rounded. 



Left valve gently convex below, moderately gibljous al)Ove. Right valve 

 Hat 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 

 depression. Wing large, joining the body lielow 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 



