Lamellibranchiata. 397 



incurved over the cardinal line. Umbonal angle acute. • Anterior 

 end short, nasute ; extremity acute. Wing very narrow-triangular, 

 scarcely defined, joining the body at the posterior extremity ; mar- 

 gin oblique, in direct continuation with the posterior extremity and 

 curving backward just beneath the cardinal line ; extremity acute. 



Test marked by concentric striae of growth which are strongly 

 fasciculate on the anterior end and ventral margin . 



The left valve described has a length of 38 mm., height 21 mm ., 

 and hinge-line 33 mm. 



This species is distinguished from the other forms of this genus by 

 its narrow wing and shorter hinge-line. It has the characteristic 

 aspect of Pteronites, although approaching, in several particulars, to 

 some species of Leptodesma. 



Formation and locality. In the Chemung group, Mansfield, Tioga 

 county, Pa. 



PAL^EOPINNA, Hall. 

 Pal^opinna flabella. 



PaloBopinna flabella, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 25, fig. 18. Jan., 1883. 



Shell large, broadly semi-ovate, gaping at the anterior extremity ; 

 axis oblique to the hinge at an angle of about 30°; length nearly 

 twice the height ; anterior margin abruptly truncate ; ventral margin 

 very broadly curving ; posterior margin oblique, sub-truncate. 

 Left valve depressed-convex in the posterior and ventral portions, 

 becoming more convex above and moderately gibbous in the umbonal 

 region. Eight valve unknown. Hinge-line straight, equaling the 

 greatest length of the shell. Beak anterior, terminal, prominent, 

 directed forward, scarcely rising above the cardinal line. Umbonal 

 region subtending an acute angle. Anterior end abruptly truncate; 

 margin excavate. Wing continuous with the body of the shell 

 without limitation or interruption ; margin directed backward, 

 nearly straight, continuous with the post-basal margin of the body; 

 extremity abruptly rounded. 



Test thin, marked by regular concentric stride of growth which 

 are cancellated by fine sub-equal radii ; in the cast the concentric 

 and radiating striae are of nearly equal strength. On the anterior 

 end the radii are obscure or obsolete as preserved in the cast. 

 Ligamental area narrow, marked by a single longitudinal groove 

 and a slight oblique furrow extending backward from the beak a 

 little more than one-third the length of the hinge. 



