64 



Fiprs. 26 and 27 represent a specimen from the Upper Helder- 

 berg Group, at Louisville, Kentucky, m the collection of 

 Wm. F. K. Gurley; and Figs. 28 to 30, two specimens from the 

 Upper Helderberg Group, at Columbus, Ohio, in the collection 

 of S. A. Miller. Several other specimens have been- examined 

 and the forms from Columbus, Ohio, are somewhat rounder 

 and a little more evenly elliptical than those from Louisville, 

 but otherwise, they appear to be alike and we are disposed to 

 refer them to the same species. It will be readily distinguished 

 from N. hicjna by the less peutalobate form, longer radial 

 plates and different surface ornamentation. It is, in all its 

 parts, a distinct species. It will be distinguished from N. 

 angnlaris, by the narrower azygous interradms, and much less 

 angular outline, when viewed from any direction, beside it is 

 proportionally a much longer form. 



