310 Thirty-fifth Report on the State Museum. 



actinopteria theta. 



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 Unpublished. 



Shell large, sub-rhomboidal ; body broadly ovate, oblique at an angle 

 of about 55°; length about one-fourth greater than the height ; 

 anterior margin, below the ear, nearly vertical, curving into the 

 broad base ; posterior end regularly rounded. Left valve moder- 

 ately convex, depressed below. Right valve smaller and less convex, 

 nearly fiat below. Hinge-line straight, length nearly equal to the 

 height of the shell. Beaks sub-anterior, prominent, arching over 

 the hinge-line in the left valve. Umbonal region moderately gib- 

 bous in the left valve, subtending an acute angle. Ear rounded, a 

 little oblique, defined by a nearly vertical depression. In the right 

 valve the ear is flat or concave, limited by the abrupt elevation of 

 the umbo. Wings large, in the left valve extending about three- 

 fourths the length of the body, and in the right valve joining the 

 body at the posterior end, without distinct limitation in either 

 valve ; margin moderately concave ; extremity slightly produced, 

 angular. 



Surface of left valve marked by sharp, elevated, lamellose, concen- 

 tric striae, with intermediate finer lines ; and by slender, elevated 

 rays, with wider interspaces between the primary rays, which are 

 often occupied with one or two finer lines; the ear and wing are 

 marked in a similar manner. The rays are distinctly crenulated 

 and the interspaces are cancellated by the concentric lines, which 

 are slightly curved backward. The surface of the right valve is 

 marked by strong, lamellose, concentric striae, which become less 

 conspicuous on the wing. The posterior slope of the body and the 

 wing of this valve are marked by numerous fine rays. 



A specimen with the two valves attached has approximately a 

 length of 48 mm., height 30 mm., and hinge-line 33 mm. 



This species resembles A. eta, but differs in the more extended 

 wing, finer radii of the left valve, with interstitial additions, narrower 

 interspaces, and more numerous concentric lamellae. 



Formation and locality. In the shales of the lower part of the Che- 

 mung group at Ithaca, N. Y. 



