LAMELLIBRANCHIA TA. 
253 
MYTILARCA, Hall. 
Sub-Genus PLETHOMYTILUS, Hall. 
Mytilarca (Plethomytilus) arenacea. 
PLATE XXX, FIG. 1 ; and PLATE LXXXvII FIG. 10. 
Mytilarca arenacea, Hall. Prelim. Notice Lam. Shells, etc., pt. 2, p. 20. Dec., 1869. 
“ “ “ S. A. Miller in Cat. Amer. Pal. Foss. 1877. 
Plethomytilus “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 30, fig-. 1. Jan., 1883. 
Shell of medium size; body ovate, acute at the apex, slightly oblique; height 
equal to about three-fourths of the length. Byssal area concave; margin 
straight below to a point three-fourths of the length from the apex ; abruptly 
curving to the posterior and basal margins; post-basal and dorsal margins 
gently curving to the extremity of the hinge-line. 
Valves moderately convex below, becoming gibbous in the umbonal region, 
and upon the anterior umbonal slope. 
Hinge-line straight and short. Beaks anterior, prominent, rising above 
the cardinal line, moderately incurved. Umbonal angle acute. Posterior 
alation narrow and short, merging into the general contour of the margin 
above the middle of the length of the shell. 
Test thick, marked by fine concentric strise of growth, which at some¬ 
what regular intervals are crowded, and raised into imbricating lamellae, as 
shown in casts of the interior. 
A valve of the ordinary size measures about 60 mm. in length, and 45 
mm. in its greatest height. 
The fossils of this species are known only as casts of the interior, in an 
arenaceous matrix, and are usually more or less imperfect, the narrow 
posterior alation being rarely preserved in connection with the body of the 
shell. 
Formation and localities. In the Schoharie grit at Schoharie, and in the 
Helderberg mountains, Albany county, N. Y. 
