364 PALAEONTOLOGY OF ILLINOIS. 



X 



rim ; four of them are equal, the posterior one is larger, and con- 

 fluent with the anal aperture. Hydrospires and summit plates un- 

 known. 



The ornamentation consists in fine concentric lines following the 

 general contour of the plates. 



Of the two specimens from which this species is described, one is 

 nearly perfect and larger than the other. On one side of the smaller 

 one is a crushed mass of slender arms, composed of pieces of about 

 equal width and height, which possibly may be some of the pinnules. 



Pentremitidea Americana resembles very closely Pentremites clavatus, 

 Schultze Pentremitidea clavata, Eth. and Carp. ; so closely, indeed, 

 that it might almost be taken for it ; the latter, however, has com- 

 paratively longer basals, is more pentalobate, has wider limbs, with 

 decidedly convex sides in place of almost straight ones, and the am- 

 bulacra extend beyond the general surface. 



Geological position, etc.: In the upper portion of the Hamilton 

 group in the Thunder Bay region of Northern Michigan. The types 

 are in my own collection. 



