878 THE PALEONTOLOGY OF MINNESOTA. 
(Tetranota sexcarinata, 
finally a narrower groove which slopes down to the angular or sharply rounded 
side of the volution. While the central pair of ridges increases in prominence, the 
lateral pair becomes more and more indistinct on the last volution, till at the aper- 
tural margin they are scarcely distinguishable,—at any rate this is true of casts. 
The transverse surface markings are prominent, regular, visible to the unassisted 
eye, about three in 1 mm.; the course of the strie from the umbilicus is at first 
nearly straight across, and it is only in the centro-lateral grooves that they 
curve backward very strongly. When the characters of the external layer are 
preserved, these very fine short lines are to be seen crossing the transverse lines 
rectangularly. 
Variety MINOR, 7. var. 
(Not figured.) 
This subordinate name is proposed for a small variety of the species which we 
have found in the Black River shales of Minnesota. The largest specimen seen is 
less than 10 mm. in hight. All four of the dorsal ridges retain their prominence to 
the apertural margin, and in this respect the variety is like 7. sercarinata. The 
edge of the umbilicus, however, is not angular, but narrowly rounded, while the 
transverse striz are much finer, eight or ten occurring in the space of 1 mm. On 
the best specimen each of the transverse lines carries a row of very minute 
prominences, 
Although we have not seen the original types of this species, we are still 
reasonably confident that the specimens above described are of the form to which 
the name didorsata should be restricted. That authors and collectors have included 
under this name more than one species is clear from published lists, they having no 
doubt viewed the dorsal ridges as a specific feature, while we regard them as of 
generic importance. 
Formation and locality.—Stones River group (Central limestone), Murfreesboro, Tennessee; Black 
River group (Ctenodonta bed,—var. minor), Minneapolis, St. Paul, Cannon Falls, and near Fountain, Min- 
nesota; Trenton group (Clitambonites and ?Fusispira beds), St. Paul, Cannon Falls, and near Fountain, 
Minnesota. In Canada Billings catalogues it as a Black River and Trenton fossil; in Kentucky and Ten- 
nessee it occurs in the same groups. Hall’s original types are from the lower beds of the Trenton at 
Middleville and Watertown, New York. Testiferous examples are very rare in Minnesota, but casts are 
rather common in the Clitambonites bed at several points in Goodhue county. 
Collections.—Geological and Natural History Survey of Minnesota; E. O. Ulrich; W. H. Scofield: 
Museum Register, Nos. 7382, 7435, 7439, 7456, 7513, 7522. 
TETRANOTA SEXCARINATA, 72. Sp. 
PLATE LXV,}FIGS. 3—9. 
This species grew to a larger size than Hall’s 7. bidorsata, the average hight 
being from 25 mm. to 30 mm. The volutions also are somewhat wider, the hight 
