25G PALAEONTOLOGY OF NEW YORK. 



Mvtilarca (Plethomytilus) Knappi, 11. sp. 



PLATE LXXXVII, FIG. 13 



Shell of medium size : body ovate, obtuse, broad below ; height nearly equal 

 to the length ; ventral margin rounded with a slight sinuosity at the byssal 

 opening: dorsal margin broadly rounded. 



Right valve moderately convex below, somewhat more convex in the 

 umbonal region. Left valve unknown. 



Hinge-line less than the height of the shell. Beak obtuse. 



Test thin below, thicker on the upper part and cardinal line. Surface 

 marked by fine, regular, elevated striae of growth. 



Ligamental area narrow, coarsely striated longitudinally. 



Interior unknown. 



The right valve described has a length of 48 mm. and height 44 mm. 



This species differs from M. (P.) oviformis in its proportionally greater height, 

 loss attenuate beak, lesser convexity and the coarsely striated ligamental area. 

 The relations of this species are somewhat obscure, owing to the obscure limi- 

 tation of the beak and the effects of maceration and compression. It is 

 however clearly distinct from any of the forms here described. 



Formation and locality. In the shales of the Hamilton group on the shores 

 of Skaneateles lake, N. Y. 



MYTILARl'A PYRAMID ATA. 

 PLATE LXXX, FIGS 1 ::. 

 Mytilarca.pyramidata, Hall. Pal. N. Y.. vol. v, pt. 1. Plates and Explanations : PI. 80, tigs. 1-3. Jan., 1SS3. 



Shell of medium size ; body sub-angularly ovate, pyramidal ; length one-third 

 greater than the height; ventral margin truncate and concave, curving 

 abruptly into the basal margin ; dorsal margin very gently curving to the 

 beaks. 



Valves equal, convex, gibbous above, and sub-angular along the anterior 

 umbonal slope. The ventral side is rectangularly incurved. 



