FOSb'ILS OF THE NIAGARA GROUP. 393 



Pleukotomaria (Trochonema) hoyi, Hall. 



PLATE XV, FIG. 10; PLATE XXV, FIGS. 11, 12. 



Phjirotomaria hoyi, HALh. Report of Progress Geol. Survey Wisconsin for 1860, p. 35. 1861. 



Shell broadly depressed-conical, the spire moderately elevated, consisting 

 of about four volutions which are gradually enlarged from the 

 apex, the last becoming slightly ventricose towards the aperture. 

 Volutions flattened upon the upper side, and the entire height of 

 each one showing above the other ; periphery somewhat flattened, 

 with a depressed band truncating the upper angle. Lower side of 

 volutions flattened, except the outer half of the last one, which is 

 rounded towards the aperture, and abruptly descending into the wide 

 umbilicus. Surface finely striated on the lower side of the volution, 

 with a deep retral curve on the band, where the strise are somewhat 

 fasciculate. The periphery of the cast is sometimes marked by three 

 or four strong but obscure stria3 below, and parallel to the revolving- 

 band. 



This species resembles P. umhilicata of the Trenton limestone, but the 

 volutions are more elevated above each other, and the upper surface is 

 wide and flat. 



Formation and Localitij. — -Limestone of the Niagara group, at Racine, 

 Wisconsin. 



Pleurotomaria idia, n. s. 



PLATE XV, FIGS. 15, 16. 

 Phurotomaria idia, Hall. Report of Progress Geol. Survey Wisconsin for 1860, p. 35. 1861. 



Shell depressed orbicular, moderately convex above, and broadly umbili- 

 cate beneath, with four volutions, which are moderately convex 

 above, and gradually increase in size from the apex, the outer half 

 of the last one being more ventricose and regularly rounded on the 

 periphery, and curving into the broad umbilicus ; aperture, or sec- 

 tion of volution near it, broadly ovate. Surface characters unknown. 



This species differs from the last in being less elevated, in its more 

 gradually increasing volutions, broader umlailicus and absence of angu- 

 larity on the periphery. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Racine, Wisconsin. 

 Cab. Nat. 50 



