548 THE PALEONTOLOGY OF MINNESOTA. 
[Oyrtodonta grandis. 
Shell large, moderately ventricose, slightly oblique, the outline subcircular. 
Beaks small, projecting slightly above the hinge line, obliquely incurved, almost in 
contact; situated about in the middle of the anterior half of the cardinal margin. 
Umbonal ridge inconspicuous, the slope of the surface to the postero-cardinal mar- 
gin gradual and slightly concave, the slope to the basal and anterior margins very 
gently convex; point of greatest convexity a little above the center of the shell. An- 
terior end longer in front of the beaks than usual in this genus, the margin narrowly _ 
rounded above, then with a very gentle and almost uniform downward and backward 
curve, merging imperceptibly into the basal, and later into the posterior margin. 
Antero-cardinal edge slightly produced; likewise the postero-ventral, but in most 
cases so gently as to be scarcely appreciable. Surface smooth, with fine concentric 
lines of growth. 
Ligamental area deep but appearing narrow in a cardinal view. As usual, it is 
finely striated longitudinally. Hinge plate strong, with three anterior teeth in the 
left, and four in the right valve. These teeth are short and abruptly curved down at 
their posterior ends, terminating with a knob-like projection. In the right valve 
the first and fourth are much smaller than the second and third; the middle tooth of 
the three in the left valve is also much the largest. Posterior teeth longer, parallel, 
and slightly curved, three in the right valve and two in the left. Anterior and pos- 
terior muscular scars large, situated just beneath the two sets of hinge teeth, the 
posterior one rather faintly impressed, the anterior deep. Pallial line simple, only 
the anterior half sharply defined, and often emphasized by a series of small pits. 
Inner side of shell with two low, subparallel ridges extending from the beaks two- 
thirds the distance to the postero-basal margin. The furrow between these appears 
as a low ridge on casts of the interior. These often present another but much 
smaller ridge running downward from the inner margin of the anterior muscular 
scar. In casts the beaks are flattened, very prominent, not strongly incurved nor 
far apart. . 
The above description does very well for the large Kentucky types upon which 
the species was founded. It is also wide enough to include a few of the numerous 
casts that occur in the lower and middle beds of the Galena of Minnesota. The 
majority of them, however, seem to fall more nearly under the variety or species 
germana which was established (Joc. cit) for the reception of usually smaller specimens » 
in which the form is a little more oblique and the hinge plate proportionately thin- 
ner and longer, with the cardinal teeth less curved and the posterior teeth placed 
more nearly horizontal. The internal furrow and the anterior, muscular scar, are 
both less deep in this variety, than in the true grandis. The same is true of the 
pallial line. 
