LAMELLIBRANCHIATA. 23 



Ligamental area narrow, well-marked, its width is .5 nun. in a specimen of 

 20 mm. in height. 



The original specimen of this species is :iu nun. in height, 18 nun. in 

 length, and the hinge-line L3 mm. Another specimen is L3 nun. in height, 

 11 mm. in length, hinge-line 8 nun. The largest specimen found has a 

 height of 22 mm. 



This species resembles A. celsus more nearly than any other form here 

 described; it differs from that one by its much smaller ears with concave 

 margins, more obtuse umbo, regular, equal striae, and comparatively shorter 



form. The two species agree in the non-obliquity of the body of the shell to 

 the hinge-line. This species lias a vertical range from the middle to the upper 

 members of the Chemung group, hut is not abundant. 



Formation and localities. In the Chemung group at Painted Post, Steuben 

 county; Chemung Narrows, Tioga county, X. Y. : and in Bradford, Tioga mid 

 Warren counties, Pennsylvania. 



AVICULOBECTEN CELSUS. 



PLATE \ II, KH.- .".', 30. 



Avleulopecten "'his. Hall. Pal. N. V.. vol. v, pi. 1. Plates and Explanations: PI. 7. figs. 29, 80. 



.Ian.. 1883. 

 Not Avicvlopecten iiltus. White. Bull. 0. B. Geol. Surv., vol. v. p. 110. 1870. 



SHELL small, ovate cuneiform, Bub-equilateral, not oblique; heighl one-fifth 

 greater than the length; pallia] margin rounded to near the ears, where the 

 sides of the shell are nearly straight. 



Right valve convex; the greatest convexity being equal to one-fourth the 

 length of the hinge-line. Left valve not observed. 



Hinge-line straight, length three-fourths the length of the shell, not extend- 

 ing as far as either lateral margin. 



Beak acute, straight, extremity pointed, arching over the hinge-line; umbo 

 prominent, subtending an angle of S0°. 



Ears broad-triangular, large. Hat, sub-equal, the margins Blightrj convex; 

 extremities rectangular; separated from the body of the shell by a moderate 



