254 
PALEONTOLOGY OF NEW YORK. 
Mytilarca (Plethomytilus) ponderosa. 
PLATE XXX, FIGS. 2-7. 
Mytilarca ponderosa, Hall. Prelim. Notice Lam. Shells, etc., pt. 2, p. 21. Dec., 1869. 
“ “ Hall. S. A. Miller, Cat. Amer. Pal. Foss. 1S77. 
Plethomytilus ponderosus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 30, tigs. 2-7. 
Jan., 1883. 
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 80 mm. A small example has a length of 36 mm. 
and height 23 mm. 
This species differs from Mytilarca (P.) arenacea in its greater size and gib¬ 
bosity ; from M. ( Plethomytilus ) cordiformis, Hall =M. (P.) myilimera , Conrad, 
and M (P.) ovata , Hall, from the Lower Ilelderberg 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, N. Y.; Clarence Hollow, Erie county, 
N. 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 is 
at right angles to this. The breadth represents the convexity of the valves. 
