870 THE PALEONTOLOGY OF MINNESOTA. 



[Protowarthla granlstrlata. 



limestones of Minnesota which may represent other varieties of the species, but as 

 we cannot be certain about them we prefer to leave them unclassified for the 

 present. 



Formation and locality. Bather a common fossil of the Stones River group, at Mineral Point, Janes- 

 ville and Beloit, Wisconsin, and Dixon, Illinois. In Minnesota it occurs, though not abundantly, in the 

 Vanuxemia bed at Minneapolis, St. Paul and Cannon Falls. 



Collection. E. O. Ulrich. 



PROTOWARTHIA GRANISTRIATA, n. sp. (Ulrich.) 



PLATE LXIII, FIGS. 28-30. 



Shell scarcely reaching the medium size, closely coiled, leaving no umbilicus; 

 center of dorsum raised into a low broad ridge, defined on each side by an obscure 

 wide furrow; with age the outer boundaries of the latter increase gradually in 

 distinctness, the back of the outer half of the last volution in the largest specimens 

 presenting a flattened appearance; but the central ridge, though decreasing some- 

 what in bight, continues to the aperture. In casts of the interior there is a small 

 umbilicus, while the central ridge is nearly as on the shelf itself. Aperture trans- 

 verse, about twice as wide as high, the width generally equalling the hight of the 

 shell; sinus wide, only moderately deep, the margin of the lobes bending rather 

 sharply where the apertural margin is intersected by the faintly raised boundaries 

 of the flattened dorsum. Except in the umbilical regions the test is thin. Out of 

 nearly thirty specimens, only two preserve anything of the external layer. These 

 show that it is marked by fine lines of growth and by very delicate revolving lines. 

 All of the other testiferous examples preserve only the inner and middle layers, the 

 latter appearing in every case quite smooth. Most of the specimens preserve what 

 may be called a fourth layer. This seems to have been deposited by the inner 

 mantle over the inner volutions, including the smaller half of the outer, while on 

 each side it extends around the callous filling of the umbilicus. The whole of this 

 layer is finely granulose, except the lateral extensions and these are covered by 

 wavy revolving striae. Hight of an average shell 19 mm.; width of aperture 19 mm.; 

 median hight of same 9.3 mm.; width of inner volution 6 mm.; depth of sinus 5 mm.; 

 width of same about 10 mm. 



In this species the sinus is only about half as deep as in P. rectangularis, the 

 apertural lobes are not rectangular, the umbilicus is closed entirely, and the 

 volutions rounded rather than subtriangular in cross section. It is nearer P. cancel- 

 lata Hall, but that species has a slightly deeper sinus, and a rounder back, the back 

 never being flattened, nor is there ever a sign of the low dorsal ridge and obscure 

 furrows characterizing P. granistriata. 



