35 



twelve moderately strong revolving ridges, which are smaller 

 and more closely arranged towards the suture (where there is 

 a slightly depressed or flattened space.) These are crossed by 

 numerous less strong, closely arranged transverse striae, which 

 bend backwards from the suture and have a strong retial curve 

 on the narrow concave band of the peripery. 



Surface characters of the under side undetermined. 



This species is so entirely distinct in its form snd surface 

 characters from any other species yet known in our Silurian 

 rocks, that it can be readily distinguished. It is more nearly 

 allied with forms in the upper Helderberg and Hamilton groups 

 of New York. 



Geological Formation and Locality. In limestone of the age 

 of the Niagara group of New York, at Racine, Wisconsin. 



Collector. T. J. Hale. 



PLEUROTOMARIA idia, (n. s.) 



Description. Shell depressed orbicular, moderately convex 

 above, and broadly umbilicate beneath, with about four volu- 

 tions, which are moderately convex above and gradually in- 

 crease in size from the apex, the outer half of the last one be- 

 ing more ventricose and regularly rounded on the periphery, 

 and into the broad umbilicus; aperture or section 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 umbilicus and 

 absence of angularity on the periphery. 



Geological Formation and Locality. In limestone, of the 

 age of the Niagara group, at Racine, Wisconsin. 

 Collector. T. J. Rale. 



PLEUROTOMARIA hoyi, (n. s.) 



Description. Shell broadly sub-conical, the spire moderate- 

 ly elevated, consisting of about four volutions, which are grad- 

 ually enlarged from the apex, the last one becoming somewhat 

 ventricose towards the aperture. Volutions flattened upon the 

 upper side, and the entire hight of each one showing above 

 the other; periphery somewhat flattened with a depressed band 

 truncating the upper angle. Lower side of volution flattened, 

 except the outer half of the last one, which is rounded towards 

 the aperture, and abruptly descending into the moderately wide 

 ambilicus. 



