LAMELLIBRANCHIATA. 125 



strongly limited. Wing comparatively large, extending three-fourths the 

 length of the valve, not strongly defined ; margin concave ; extremity -pvo- 

 duced 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 stria?, and by 

 radiations with wider interspaces, which have sometimes slender intercalated 

 rays. The concentric lamellse crenulate the radii, and are arched backward 

 in the interspaces, producing a beautifully cancellated surface. The finer 

 concentric striae between the lamellse are rarely visible, and the spaces 

 between tlie rays show, under a lens ami in well-preserved examples, 

 extremely fine, ci'owded radiations. The radii are also seen in the depression 

 limiting the ear, and on the wing they are equal to those on the l)ody of 

 the valve. The right valve is marked l>y regular, equidistant, 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. 



Interior unknown. 



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 distinguished by 

 its greater obliquity and much smaller wing. 



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

 N. Y. 



ACTINOPTERIA THETA, n. sp. 

 PLATE LXXXIV, PIGS. 18, 1». 



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 ))i-oad base; posterior cml 

 regularly rounded. 



