386 THE PALEONTOLOGY OF MINNESOTA. 
IStrophomen ucurvata. 
the mouth of Green bay, as well as near Navarino [now Green Bay city.'Wisconsin], 
at the head of the bay. In the last mentioned region it abounded in a species of 
Produda, which I take to be undescribed and shall, therefore, denominate the 
incurvata. Specific character: Semi-circular; hinge nearly straight and the length 
of the shell; with fine longitudinal striae; flattish; edge crenated; shallow valve, 
concave, basal margin incurved; muscular impressions and hinge-process very dis- 
tinct. The space between the valves is very small in this species." 
Shell moderately large, more or less strongly concavo-convex, semi-oval, wider 
than long, with the greatest breadth along the hinge-line; cardinal extremities 
acutely angular and deflected, lateral and anterior margins regularly rounded. Sur- 
face marked by numerous, fine, subequal, crowded and rounded, radiating striae, 
increasing by intercalation, with every second, third or fourth one more prominent. 
This alternation in the size of the striae is variable; it is nearly obsolete in some, 
while in others it is a prominent feature, the whole surface being crossed by numer- 
ous and crowded, very delicate, raised, concentric lines and a few stronger marks of 
growth. Sometimes there are some oblique wrinklings along the cardinal margin 
on each side of the beaks. 
Ventral valve slightly convex in the umbonal region, but otherwise more or less 
deeply concave; point of greatest elevation at the beak, which is minutely perforated 
for, the passage of the pedicle. Cardinal area variable in width and elevation, 
slightly retrorse in very gibbose specimens or strongly elevated in flattish speci- 
mens; deltidium conspicuous, broadly convex, as wide as, or wider, than long, 
broadly excavated anteriorly and entirely occupied by the chilidium. The teeth 
are divergent, not very prominent and attached to the much elevated outer margin 
of the large, strongly striated, suboval muscular area. In the center of the posterior 
half of this area there are two slender, short adductor scars, which are separated by 
a septum attaining its greatest development toward the anterior margin, upon each 
side of which are the large impressions of the diductors, and probably also of the 
a-Ijustors. On either side of the muscular area there are numerous, elongated 
tubercles, arranged in more or less regular radiating series, the genitalia markings. 
Near the outer margin of the valve is a more or less strongly defined, concentric, 
irregular thickening, crossed in the anterior region by a number of short, irregular, 
vascular sinuses. The whole of the interior is minutely granulose. 
Dorsal valve flattened or slightly concave in the umbonal region, and more or 
less strongly convex laterally and anteriorly; sometimes there is a shallow, narrow 
sinus present, becoming obsolete before reaching the anterior margin. Cardinal 
area narrow, vertical, centrally occupied by a very broad but short convex chilidium. 
Dental sockets] conspicuous, rapidly narrowing and continued as grooves over the 
