LAMELLIBRANCHIATA. V7 
Plethocardia suberecta.] 
the antero-cardinal margin of the shell; posterior scar indistinct, larger than the 
anterior, situated a short distance beneath the post-cardinal margin. Pallial line 
faint, simple, submarginal. 
It is possible that this species is not distinct from the Cyrtodonta cordiformis of 
Billings. His figures of that species looks so much like the shell above described 
that I am nearly satisfied that they must be congeneric at least. It might be a. 
Whitella, but it is not a true Cypricardites. Compared with P. umbonata it appears 
that in the Canadian shell the beaks are situated farther back from the anterior 
extremity, the umbonal ridge is rounded instead of angular and the outline different, 
especially that of the posterior end, which is also wider. 
Formation and locality.—Upper part of the middle third of the Trenton shales six miles south 
of Cannon Falls, Minnesota. Also in cherty limestones of the age of the Black River limestone of 
New York, in Mercer county, Kentucky. 
PLETHOCARDIA SuBEREOTA Ulrich. 
PLATE XL, FIGS. 25—27. 
Plethocardia suberecta ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 245. 
” 
Shell small, but little oblique, exceedingly ventricose, short, subelliptical in a 
side view, with the dorso-ventral diameter much the longest. Beaks very prominent, 
large, strongly incurved, nearly terminal; umbonal ridge strong, sharply rounded, 
with the cardinal and posterior slopes very abrupt and nearly flat. Anterior end 
very short, the part in front of the beaks of casts consisting chiefly of the sharply 
defined lobe-like fillmg of the anterior muscular impressions. Anterior and poste- 
rior margins gently convex, subparallel; ventral edge sharply rounded. Hinge line 
short, scarcely extending posterior to the umbonal ridge, as seen in a side view. In 
the casts there is a depression beneath each beak that is prolonged on each side 
around the muscular scar. The escutcheon seems to have been narrow, but the 
internal ligament supports at the posterior end of the hinge line have left two 
strong grooves, one on each side. 
This species, though clearly congeneric with P. wmbonata, is so readily distin- 
guished that comparisons are unnecessary. 
Formation and locality.—Galena shales near Cannon Falls, Minnesota. 
