878 THE PALEONTOLOGY OF MINNESOTA. 



[Tetranota sexcarluata. 



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 striae 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, n. 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 bight. All four of the dorsal ridges retain their prominence to 

 the apertural margin, and in this respect the variety is like T. sexcarinata. The 

 edge of the umbilicus, however, is not angular, but narrowly rounded, while the 

 transverse striae 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 bidorsata 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 Rtver group (Central limestone), Murfreesboro, Tennessee; Black 

 River group (Ctenodonta bed, var. miiwr), Minneapolis, St. Paul, Cannon Falls, and near Fountain, Min- 

 nesota; Trenton group (Clitambonites and ?Fusisplra 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. Scofleld. 

 Museum Register, Nos. 7382, 7435, 7439, 7456, 7513, 7522. 



TETRANOTA SEXCARINATA, n. sp. 



PLATE LX V.I FIOS. 3-U. 



This species grew to a larger size than Hall's T. bidorsata, the average hight 

 being from 25 mm. to 30 mm. The volutions also are somewhat wider, the hight 



