The Fauna of the Magnesian Series. 99 



surface of the volution, forward again down to the base and thence with a 

 back and forward curve across the umbilical surface. 



Found in the Oneota dolomite near Dresbach, at Altura and Man- 

 kato, Minnesota, near the top of the formatiom. Also at Caledonia, Hous- 

 ton county. 



Plarotomaria sweeti Whitfield. 



Holopea svreeti Whitfield, Geology of Wisconsin, vol. iv, p. 174, pi. X, 

 fig. 3, 1882. 



Casts of this shell occur at Osceola Mills, Wisconsin, in the Jordan 

 sandstone. They agree with Mr. Whitfield's description and the figure of 

 his specimen except that the growth lines on casts of theexteriorshow that 

 the aperture had a very deep sinus which has left on some an elevated, 

 broadly rounded band just below the middle of the outer surface of the last 

 volution. This species is of the same tj'pe as the Pleurotomaria aiens Sar., 

 from the Saint Peter sandstone. 



Rapbistoma leiosomellum n. sp. 

 Plate V, figures 7 and 8. 



The shell of this species, as shown by the quartz casts found was 

 small and consisted of about four rapidly increasing volutions, which em- 

 brace in such a manner as to form a lenticular coil. Each volution conceals 

 about one-half of the surface of the preceding one. The sutui-e is channelled, 

 the periphery is marked by a rounded somewhat swollen band, and the um- 

 bilicus,. which is about one-fourth the entire width of the coil, is abrupt and 

 ascends by degrees quite to the apex. The surface is smooth and nearly 

 equalh' convex above and below. 



From the upper portion of the Oneota dolomite near Dresbach, at 

 Altura, Winona county, and near Caledonia, Houston county, Minnesota. 



Rapbistoma lewistonense n. sp. 

 Plate V, figures 9 and 10. 



The only specimen of this species found is a hollow cast of the exterior 

 of a shell, and from this rubber casts have been taken. These show a coil of 

 about four volutions which are flat above and strongly ventricose below. 

 The suture of each volution falls a little below the periphery of the preced- 

 ing volution. The umbilicus is about one-third the entire width and as- 

 cended probably to the apex. Thevaricesof growth appearto run obliquely 

 back from the sutures to the periphery and thence directly down and into 

 the umbilicus. 



Found in the Oneota dolomite near Lewiston, Winona county, Minne- 

 sota. Identified also at Shakopee and Cannon Falls, Minnesota. 



