372 A. W. GRABAU — SILURO-DEVONIC CONTACT IN NEW YORK 



is usually very common, often crowding the surface of the thin layers of 

 limestone. The general outline of the carapace is bean-shaped, as in 

 Leperditia generally. The greatest height is posterior to the middle. 

 Hinge line straight, about two-thirds the length of the carapace, termi- 

 nating anteriorly in an obtuse, slightly salient angle. Posterior extrem- 

 ity of hinge line likewise salient, with the posterior border below it 

 uniformly rounded on a short radius. Anterior dorsal margin sloping 

 off abruptly, making an angle of about 130 degrees with the hinge line. 

 Anterior end nasute, obtusely rounded. Basal margin a uniform hut 

 asymmetric curve, more convex in the posterior portion of the shell. 

 A distinct marginal border or fold occurs on both anterior and posterior 

 ends, the former being the stronger and best defined. It is well flat- 

 tened, with the margin sometimes slightly elevated. Ocular tubercle 

 about a third the length of the carapace from the anterior end and 

 about a fourth of the height below the dorsal margin. The longitudinal 

 contour is a flattened curve, rather more convex in the anterior third 

 and becoming abrupt near the ends. Dorso-ventral contour an asym- 

 metric curve, flatter near the hinge line and abruptly incurved at the 

 ventral border. The ventral border of the right valve overlaps that of 

 the left valve, which is abruptly flattened. 



In the left valve occurs a strong, elongated fold or nodule, situated 

 just below the hinge line in the posterior half of the carapace. It begins 

 about midway of the length of the hinge line and extends backward to 

 half way between the center and the posterior end, thus equaling in 

 length about a fourth of the hinge line. This fold is accentuated by 

 an abrupt depression of the valve below it, the fold thus becoming 

 strongly pronounced below, but grading into the upper slope of the 

 valves. This fold or " dorsal hump " is wanting in the right valve. 

 Surface smooth. A perfect right valve measures: Length, 11.5 milli- 

 meters; height, 7.5 millimeters; hinge, 9 millimeters; greatest con- 

 vexity, 2.5 millimeters. Another measures 12 by 7 millimeters, with 

 hinge line 8 millimeters long. Another measures 11.5 by 6.5 millimeters. 

 Three left valves from Williams ville measure respectively 10.5 by 5.5 

 millimeters, 9.2 by 5 millimeters, and 8.5 by 4.5 millimeters. 



Specimens of similar form occur, according to Jones, in the black 

 limestone of the " Scalent group " of Pennsylvania. The left valve has 

 ■a distinct small dorsal hump, the right valve being without it. 



This species recalls in form and size L. jonesi Hall of the Coralline 

 limestone of Schoharie. No dorsal hump has been described or figured 

 for this species, but Jones saj's : * 



* A nun Is and Magazine of Nat urn I ll istory, ">t li series, vol. 1 1. j>. 343. 



