LAMELLIBRANCHIA TA. 
125 
strongly limited. Wing comparatively large, extending three-fourths the 
length of the valve, not strongly defined; margin concave; extremity pro¬ 
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 striae, and by 
radiations with wider interspaces, which have sometimes slender intercalated 
rays. The concentric lamellae crenulate the radii, and are arched backward 
in the interspaces, producing a beautifully cancellated surface. The finer 
concentric striae between the lamellae are rarely visible, and the spaces 
between the rays show, under a lens and in well-preserved examples, 
extremely fine, crowded radiations. The radii are also seen in the depression 
limiting the ear, and on the wing they are equal to those on the body of 
the valve. The right valve is marked by 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 LXXX1V, FIGS. 18, 19. 
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. 
