LAMELLIBRANCHIATA. 259 



Mytilarca carinata. 



PLATE XXXII, FIGS. 15-19 ; AND PLATE XXjaH, FIG. 8. 



Mytilarca carinata. Hall. MSS. 1877. 



" " " S. A. MiLLEK, in Cat. Amer. Pal. Foss., p. 197. 1877. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 33, fig-s. 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 strias 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 diflers 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. 



