LAMELLIBRANCHIATA. 331 



Beaks between the anterior third and fourth of the length of the shell, 

 prominent, rising above the hinge-line. Umbonal ridge distinctly defined, 

 sharply angular above, becoming truncate below in its extension to the 

 posterior extremity. 



Surface marked by fine, regular, equal, sharp concentric strise, which con- 

 verge upon the posterior end of the shell, cross the umbonal angle, and are 

 obsolescent on the post-cardinal slope. 



Hinge crenulated with minute teeth which extend half the distance from 

 the beak to the posterior end. 



Three specimens measure respectively 16, 13 and 10 mm. in length, and 

 6.5, 6, and 4.5 mm. in height. 



This species differs from L. diversa in its more elongate form, in the more re- 

 curved and falcate posterior extremity, proportionally more anterior beaks, finer 

 and more regular stria), which do not become confluent on the umbonal ridge. 



Some of the specimens are marked by extremely fine striae, which are scarcely 

 visible except under a magnifier. This character, and their somewhat shorter 

 form, led to their separation under the name of L. persiriata, but the distinction 

 is found not to be tenable, as in some specimens the striae are extremely fine in 

 the upper part of the shell, and have the characteristic appearance of typical 

 specimens of L. rostellata in the lower part. The older individuals are more 

 extremely attenuate behind than the younger specimens. 



Formation and localities. In the shales of the Hamilton group, on the shores 

 of Skaneateles, Owasco and Seneca lakes, and at Pratt's falls, Onondaga 

 county, N. Y. 



Leda obscura, n. sp. 



PLATE XLVII, FIG; 48. 



Leda (Nuculana) curta, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 47, fig. 48. 1883. 

 Not Leda curta, Mbbk. Proe. Acad. Nat. Sci. Phila. 1861. 



Shell small, ovate-triangular, acute at the posterior extremity ; length one-third 

 greater than the height. Gibbous in the umbonal region, and regularly con- 

 vex below, with a slight depression at the base toward the posterior end. 

 Beak between the middle and anterior third. Umbonal ridge not defined. 



