588 THE PALEONTOLOGY OF MINNESOTA. 



[Otenodonta glbbeiula. 



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 fulcra; 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 striae 

 of growth, tending to irregularity in the basal parts of old shells. 



Impressions of adductor muscles extremely ^eep, the anterior pair larger thali 

 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. 



I have very carefully compared a large series o^ the northwestern form which 

 Hall named ventricosa with authentic (^Janandian 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 Tellinamya ventricosa may be the same as C. contractu 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 Elver limestone at Pauquette's 

 Rapids. In Tennessee It occupies an equivalent ox 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. 



i^MS. Reg. Nos. 8309, «316, 8339. 



