LAMELLIBRANCHIATA. 253 



MYTILARCA, Hall. 



Sub-Gentjs PLETHOMYTILUS, Hall. 



Mytilarca (Plethomytilds) 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. 



Pletlurmytiltis " " Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 30, fig. 1. Jan., 18S3. 



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 concenti'ic striae of growth, which at some- 

 what regular intervals are crowded, and raised into imbricating lamellte, 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 pi'eserved 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. 



