25G 
PALEONTOLOGY OF NEW YORK. 
Mytilarca (Plethomytilus) Knappi, n. 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 iff. (P.) oviformis in its proportionally greater height, 
less 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. 
Mytilarca pyramidata. 
TLATE LXXX, FIGS. 1-3. 
Mytilarca pyramidata. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 80, tigs. 1-3. Jan., 1883. 
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. 
