880 THE PALEONTOLOGY OF MINNESOTA. 
[Tetranota obsoleta. 
TETRANOTA OBSOLETA, ”. Sp. 
PLATE LXV, FIGS. 19—23. 
This species differs from 7. bidorsata in several obvious respects. Chief among 
-these is the fact that the revolving ridges are much less developed, especially on the 
last volution, the centro-lateral pair being quite obsolete except on the inner volutions, 
while even the central pair does no more than to merely maintain the same strength 
relatively that it held in earlier stages. Interior casts of mature shells exhibit a 
broad, comparatively low and more or less distinctly grooved central ridge, beyond 
which the surface is first shallowly excavated and then gently convex to the lateral 
boundaries of the volutions, which again are not angular but narrowly rounded. The 
exterior of the shell looks the same, only the ridges bordering the slit-band appear 
thinner and sharper. Continuing our comparisons with T. bidorsata we find that the 
umbilicus is smaller and less abrupt, and the volutions more rounded on each side and 
therefore elongate-reniform in cross-section. 
The form of the aperture and the surface markings seem to be very nearly the 
same in the two species. The latter were easily abraded, and on only a single 
specimen,—it is from the Utica group at Cincinnati and doubtfully referred 
to the species,— have we been able to make them out at all. In this specimen 
they are imperfectly preserved near the aperture. Here they appear to be 
somewhat finer than in T. bidorsata and each seems to have borne a row of minute 
prominences. 
It is scarcely likely that any one will ever find it difficult to separate 7’. obsoleta 
from T’. sexcarinata, the apertural sinus being deeper and the revolving ridges even 
more prominent and constant in that species than in 7’. bidorsata. 
The largest specimens of 7. obsoleta occur in the Vanuxemia bed of the Stones 
River group at Minneapolis. One of these has a hight of nearly 30 mm. The others 
range from this size down to about 15mm. The average hight of the specimens 
from the Black River and Trenton groups is less, being only about 15 mm., while in 
the largest it did not exceed 20 mm. In the Utica group however the average was 
increased again to almost 20 mm. 
Formation and locality.—Stones River group (Vanuxemia bed), Minneapolis and St. Paul, Minne- 
sota; Janesville, Wisconsin. Black River group, Mercer county, Kentucky; (in Ctenodonta bed) Chatfield 
and six miles south of Cannon Falls, Minnesota. Trenton group (Fusispira bed), Goodhue couty, Minne- 
sota. Utica group at Cincinnati, Ohio, and localities in the vicinity of that city. 
Collections.—Geological and Natural History Survey of Minnesota; E. O. Ulrich. 
Museum Register, Nos. 510, 5109, 7294, 7465. 
