Lamellibranchiata. • 339 



arranged, and on the lower portion, in the cast, they exhibit the 

 gentle undulations of the typical forms of the right valve. 



Formation and localities. In shales of the Hamilton group, in 

 Ontario county ; Norton's Landing, Cayuga county ; Bellona, Yates 

 county, and in the coarse grits at Leonardsville, Madison county, 

 N. Y. 



Leiopteria Dekayi. 



Leiopteria Dekayi, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 19, fig. 1 ; pi. 20, figs. 16-18 (19 in error). Jan., 1883. 



Shell of medium size, sub-rhomboidal in general form ; body nar- 

 rowly ovate, oblique ; length about equal to the height; anterior 

 margin nearly vertical for one-third the length of the shell, thence 

 broadly rounded over the basal margin and abruptly recurved at the 

 post-basal extremity. Left valve convex, gibbous on the umbo. 

 Eight valve smaller and less convex ; the limitation of the wing 

 not strongly defined. Hinge-line straight, length a little less than 

 the length of the shell measured from the anterior side "of the beak 

 to the extremity of the wing. Beaks anterior, acute, prominent, 

 inclined forward. Umbonal region gibbous above, regularly con- 

 vex below, subtending an acute angle. Ear bending downward, 

 with a strong fold in the upper part, limited by a broad undefined 

 sulcus and shallow byssal sinus. Wing, triangular, flat, defined by 

 the abrupt retral bending of the striae and a depressed line along 

 the post-cardinal slope ; margin concave ; extremity acute. 



Test thin, marked by fine, regular, concentric striae, which at un- 

 equal intervals are crowded into fascicles, giving an undulated aspect 

 to the surface of the cast ; in old specimens, more crowded on the 

 anterior side ' and ear, and on the wing usually regular, sometimes 

 becoming a little fasciculate ( on the margin. This description of 

 the surface is taken from casts or partial casts which preserve more 

 or less the surface markings. In a few younger specimens from the 

 shales, the surface shows fine regular striae which gradually become 

 fasciculate as the shell increases in size. Ligamental area narrow, 

 marked by fine striae ; below this area is a narrow oblique fold or 

 lateral tooth. 



Three specimens have respectively the following dimensions: 

 length, 37, 31, and 29 mm., height, 37, 30, and 30 mm., hinge- 

 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. Gonradi and L. Rafinesquii, but 

 there are some differences in shape and obliquity and more decidedly 



