LAMELLIBRANCHIA TA. 
259 
Mytilarca carinata. 
PLATE XXXII, FIGS. 15-19; and PLATE XXXIII, FIG. 8. 
Mytilarca carinata, Hall. MSS. 1877. 
“ “ “ S. A. Milleh, in Cat. Amer. Pal. Foss., p. 197. 1877. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 33, figs. 15-19. 
Jan., 1883. 
Shell of small or medium size; body obliquely sub-ovate; length nearly twice 
the height, except in young specimens; ventral margin nearly straight or 
slightly concave for more than three-quarters of the length of the shell, 
thence rounding abruptly into the basal margin, then more gently rounded 
into the gently curving dorsal margin. 
Valves equal, depressed-convex in the posterior part, becoming moderately 
convex in the middle and scarcely gibbous above. The ventral umbonal 
slope is angular, and the shell between it and the margin is concave. 
Hinge-line short, less than the height of the shell. Beaks anterior, prom¬ 
inent, acute, slightly curving forward. 
Test thin, marked by very fine concentric striae of growth, which are 
crowded on the anterior side and toward the base. 
Ligamental area of moderate width, finely striated longitudinally. 
Cardinal teeth small, diverging, situated under the extremity of the beak. 
Lateral teeth two, small, oblique, situated just below the posterior extremity 
of the hinge-line. Other characters of the interior unknown. 
A specimen has a length of 30 mm. and height 18 mm. A more elongate 
form has a length of 43 mm. and height 20 mm. A small individual has a 
length of 23 mm. and height 15 mm. 
This form differs from M. Ckemungensis in its proportionally shorter form, 
the young shells have a sub-triangular aspect, the ventral umbonal slope is 
always angular, and the body is more distinctly arcuate. 
Formation and localities. In the middle portion of the Chemung group, Tioga 
and Chemung counties, N. Y. 
