LAMELLIBRANCHIATA. 165 



line 31, 21, and 23 mm. A smaller specimen has length of 17 mm., height 

 16 mm., hinge-line 15 mm. 



This species resembles in form L. Conradi and L. Rafinesquii, but there are 

 some differences in shape and obliquity, and more decidedly in surface 

 characters. The wing is not so much extended as in L. Conradi, the striae are 

 more abruptly recurved, and the post-cardinal slope is more strongly defined. 

 There is a general resemblance with L. Bigsbiji, but tlie bod}- ami hinge-line 

 are longer, the body narrowing more rapidly from the base, and the strise are 

 equal and uniform. 



Formation and localities. In the soft shales of the Hamilton group ; at Skane- 

 ateles and Cayuga lakes ; and in the upper coarse beds and lower shales at Pratt's 

 falls, Onondaga county ; also at Schoharie, N. Y. It is widely distrilmted from 

 the eastern to the central portion of the State. 



Leiopteria Bigsbyi. 



PLATE XX, B'IGS. 3, 11, 13-15; AND PLATE LXXXVIII, FIG. 23, 



Leuipteiia Bigxhi/i. Hall. 1'mI. N. Y., vol. v, pt. 1. Plates und Exi.laiKitiiiiis : PI. L>0, tiys. 3, II, 13-1.'. 



Jan., 1SS3. 



Shell larger than medium size, sub-rhomboidal ; body ovate, oblique ; height 

 greater than the length ; anterior margin nearly vertical for about half the 

 height of the shell, thence regularly rounded along the base ; posterior side 

 moderately extended. 



Left valve gibljous upon the nmljo, less convex towards the base. Right 

 valve gibbous on the umbo, depressed-convex below, shorter than the left 

 valve. 



Hinge-line straight, equalling or less than the length of the shell. 



Beaks anterior, acute, prominent, rising above the hinge-line, inclined for- 

 ward. Umbonal region prominent, subtending an angle of about 55°. 



Ear short, bending moderately downward, with a strong fold at the upper 

 side, limited by a broad, undefined sulcus. Byssal sinus shallow. Wing 

 triangular, wide, nearly flat ; margin concave ; exti-emity acute. The wing 



