880 THE PALEONTOLOGY OF MINNESOTA. 



[Tetranota obsoleta. 



TETRANOTA OBSOLETA, n. sp. 



PLATE LXV, PIGS. 19-23. 



This species differs from T. 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 T. obsoleta 

 from T. sexcarinata, the apertural sinus being deeper and the revolving ridges even 

 more prominent and constant in that species than in T. bidorsata. 



The largest specimens of T. 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 15 mm. 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. Ulack Kiver group, Mercer county, Kentucky; (in Ctenodonta lied) Chatfleld 

 and six miles soiitli of Cannon Falls, Minnesota. Trenton group (Kusispira 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 Minnesnta: E. O. Ulrich. 

 Museum Register, Nos. 610, 5109, 7294, 7405 



