Lamellibranchiata. 401 



MYTILARCA, Halx. 



Sub-Genus PLETHO M YTILITS, Hall. 



Mytilarca (Plethomytilus) Knappi. 



Mytilarca {Plethomytilus) Knappi, Hall. Pal. N. Y., vol. v, pt. 1. Unpub- 

 lished. 



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. 

 Eight valve moderately convex below, somewhat more convex in the 

 umbonal region. Left valve similar in general characters. 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. Liga- 

 mental area narrow, coarsely striated longitudinally. 



A right valve has a length of 48 mm., and height 44 mm. 



This species differs from 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 limitation of the beak and the effects of macera- 

 tion 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. 



Mytilarca pyramidata, Hall. Pal. N. Y., vol. v, pt 1. Plates and Explana- 

 tions: PI. 80, figs 1-3. Jan., 1883. 



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

 one-third greater than the height; ventral margin truncate and con- 

 cave, 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. Hinge-line short. Beaks acute, 

 extremely elevated, directed slightly forward and curving over the 

 cardinal line. 



[Sen. Doc. No. 38.] 51 



