254 PALEONTOLOGY OF NEW YORK. 



Mttilarca (Plethomytilus) I'ONDEROSA. 



PLATE XXX, PIGS. -'7 



Mytilarca pondeima, Hall. Prelim. Notice Lam. Shells, etc., ]>'. 'J. p. 21. Dec, 1869. 



11m.'.. S. A. Miller, Cat. Amer. Pal. Foss. L877. 

 Plethmnytihis ponderosus, Hall. Pal. N. Y., vol. v. pt. 1. Plates and Explanations: PI 30, figs. 2-7. 



Jan.. INS!. 



Shell very large; body acutely ovate, erect; height three-fifths the length;* 

 ventral margin nearly straight or slightly concave for about one-half the 

 . length of the shell ; posterior and basal margins abruptly rounded : dorsal? 

 margin gently curved to the extremity of the hinge-line. 



Valves equal, regularly convex posteriorly, gibbous above and in the 

 umbonal region. 



Hinge-line straight and short. Beaks anterior, prominent, rising much 

 above the cardinal line, moderately incurved. Ventral side concave with a 

 slight inflation of the margin at the byssal opening. Dorsal side sub-alate. 



Test thick, marked by concentric striae of growth which become lamellose 

 toward the margin. 



Ligamental area finely and evenly striated longitudinally, in old shells 

 becoming very wide, indicating great thickness of the test. 



Interior unknown. 



A large specimen, which is a cast of the interior, has a length of 135 mm., 

 height 80 mm., breadth SO mm. A small example has a length of 36 mm. 

 and height 23 mm. 



This species differs from Mytilurca (P.) arenacea in its greater size and gib- 

 bosity; from M. (Plethomytilus) cord /form is, Hall=M (P.) myilimera, Conrad, 

 and M (P.) ovata, Hall, from the Lower Helderberg group, in its more elongate 

 form and more attenuate apex. 



Formation and localities. In the limestone of the Upper Helderberg group, 



Helderberg mountains, Albany county. X. Y. ; Clarence Hollow, Erie county, 



X. Y. : and in limestone of the same age at Columbus, Ohio, and Walpole, 



Ontario. 



In the mytiloid forms the length is measured from the beak to the posterior extremity and the height i> 

 al righl angles to this. The breadth represents the convexitj of the valves. 



