2r,0 PALAEONTOLOGY OF NEW YORK. 



MyTILARCA (PlETHOMYTILUS) KnAPPI. n. Sj). 

 I'l.ATI-; I.XXXVII, KIC. 13 



Shell ul' medium .size; body ovate, obtuse, liru;nl lielow ; hei,<,4it nearly equal 

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

 opening; dorsal margin broadlx roiiiidcil. 



Ri'dit \:il\c iiioilcniti'h cdiiveN below, somewhat iiKire convex in the 

 umbonal region. Left valve unknown. 



Hinge-line less tlian tlie lieiiiht 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 i8 mm. and height 44 mm. 



This species differs from M. (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 descriljed. 



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

 of Skaneateles lake, N. Y. 



Mytilarca pyramidata. 



ri.Aii-; l.wx, I'HiS 1 :!. 

 MytVarcn.p)jram\datn.U.\\.\.. I'.il. N. Y.. vol. v, j.l 1. I'hi'.i's iin.l i;x|ilaii:itinns : PI. SO, Hirs. 1-3. .liiii., 1SS.S. 



SiiKLi- of medium size; Ijody 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 < urx ing to the 

 beaks. 



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

 umbonal .slope. The ventral side is rectangularly incurved. 



