588 THE PALEONTOLOGY OF MINNESOTA. 
{[Otenodonta gibberula,. 
truncate above the narrow and sharply rounded lower part; anterior end broadly 
rounded and continuing into the basal margin; the latter is straight or very gently 
sinuate and ascends from the prominently rounded anterior part; posterior umbonal 
ridge inconspicuous in a lateral view, rather sharply defined, however, in a dorsal 
view by a narrow furrow which outlines a wide lanceolate flattened area, equally 
divided by the hinge line, and in the upper part of which (immediately behind the 
beaks) the ligament is attached to distinct fulera; anterior dorsal slope abruptly 
rounded; entire anterior half of valves strongly ventricose, while between this part 
and the posterior umbonal ridge a slight sulcus crosses from near the beak to the 
base. Surface marked by rather distinct, closely arranged, subequal concentric strize 
of growth, tending to irregularity in the basal parts of old shells. 
Impressions of adductor muscles extremely deep, the anterior pair larger than 
the posterior. A small, though distinct, pedal muscle scar is always present on the 
upper part of the strong ridge which forms the inner boundary of the anterior 
adductor, (in casts it lies at the bottom of the deep cavity produced by this ridge), 
but the corresponding posterior scar is rarely distinguishable. Hinge plate very 
narrow at the beaks, but widening rapidly on each side, the anterior half somewhat 
the stronger and slightly concave along its inner margin, both terminating abruptly 
at the muscular scars; denticles twelve behind and ten or eleven in front, those 
near the beaks very small, all interlocking deeply, especially those of the anterior 
set, which are also somewhat larger than the posterior. The shell is very thick and 
the rostral filling so considerable that in casts of the interior the beaks appear 
obtuse and widely separated. E 
I have very carefully compared a large series of the northwestern form which | 
Hall named ventricosa with authentic Canandian examples of Salter’s C. gibberula, 
and I can say, with perfect confidence, that there is not the slightest reason for con- 
sidering them as distinct species. The only difference that I can detect is that the 
northwestern specimens grow to a larger size than the Canadian. Tennessee speci- 
mens also attain greater dimensions than the latter, though their average size 
scarcely equals that of Wisconsin examples. Meek and Worthen suggested (op. cit.) 
that Tellinomya ventricosa may be the same as C. contracta Salter, but in this they 
were mistaken, since that species is certainly distinct and, so far as known, does not 
occur at any of the northwestern localities. 
Formation and locality.—In Canada this species occurs in the Black River limestone at Pauquette’s 
Rapids. In Tennessee it occupies an equivalent or lower position near Murfreesboro. In Wisconsin, at 
Beloit, Janesville and Mineral Point, and at several localities in Illinois and Iowa, it forms one of the 
most striking fossils of the limestones beneath the Galena, particularly the ‘‘ Upper Buff.’ In Minne- 
sota it seems to be a rare fossil, being, so far, known only from Minneapolis, where it occurs in the 
Trenton limestone a few feet beneath the shales. 
Mus. Reg. Nos. 8309, 8316, 8339. 
