100 The Fauna of the Magnesian Series. 



Rapbistoma minnesotenses Owen. 

 Plate V, figures 15, 16 and 17. 



StraparoUus {Euowphalus) minnesotensis Owen (1852), Report Geo- 

 logical Survey of Wisconsin, Iowa and Minnesota, p. 581, tab. II, figs. 12 

 and 13. 



Euompbalus vaticinus H., 1863, 18th Rep. N. York Mus. Nat. Hist, 

 p., 136. 



Shell large, of eight or nine slender whorls, which gradually expand 

 and remain in contact throughout. The apical angle is very obtuse at 

 first but gradually decreases to about 120° as the coil expands, giving a 

 somewhat lenticular outline. This character also varies in specimens of 

 the same size; umbilicus very wide. The volutions are strongly convex be- 

 low and above, but gently concave along the strong angular carina upon 

 the outer side and strongly concave along the same above. The carina 

 rises above the sutures on the internal casts, but not so strongly on the ex- 

 terior of the shells. The aperture is subquddrate and deeply notched. 



This is an abundant species in the upper strata of the Oneota, but is 

 found only as imperfect specimens, the smaller ones of which are easily mis- 

 taken for a distinct species. There are, however, no others with which this 

 one could be confused known to bccur in the same formation. 



Found at Dresbach, Winona county, near Red Wing, Goodhue county, 

 at Mankato and other places in Minnesota, and Blanchardville, Wisconsin. 

 Also found in the Jordan sandstone near Rapidan, Blue Earth county, 

 Minnesota. At Red Wing also in the Saint Lawrence. 



Rapbistoma owenin. sip. 

 Plate V, figure 18. 



Shell often or twelve very slender whorls in close contact, but not al- 

 ways uniformly coiled. The apical angle of the shell varies a little from 140°. 

 The umbilicus is very wide. A transverse section of each voluiion presents 

 a quadralateral figure which is approximately a parallelogram with the 

 outer angle acute, equal to about 75°. Upon the outer angle or carina is a 

 small band which on the spire rises a little above the sutures. The upper 

 surface of a volution has a depression along the carina, is nearly flat over 

 most of the surface, but strongly rounded on top into the suture. Below, 

 the surface is flattened or gently concave on the outer and umbilical areas, 

 but has a strongly convex or subangular area between these two. In the 

 umbilicus each volution leaves exposed to v .wapartof the outer surface of 

 the preceding one, while on the apical surface the sutures are close or only 

 slightly channelled. The surface is indistinctly striated on internal casts 

 by transverse lines which on the upper surface curve back at an angle of 

 about 45°, from the suture to the carina. The irregularity in coiling does 

 not seem to be due to distortion. 



From the Oneota dolomite of the Magnesian series in the Saint Croix 

 valley, above Stillwater, Minnesota. 



