from the Si'lnritt/i Uorks of North America. 183 



Ceramojyora. W'^orn examples, however, exliibit very different 

 cliaraetcrs ; and when tlic entire ori^^inal surface has been 

 abraded, it is sometinuis difficult or impossible to determine 

 whether or not one is dealing with this or some entirely dif- 

 ferent form. 



When slightly A\'orn, the appearances shown in fig. 7 a are 

 exhibited. The delicate front wall of the cell has now dis- 

 appeared ; and the cavity of the cell appears to be divided into 

 two distinct com])artments, a larger and a smaller, both of a 

 somewhat triangular shape, by an oblique internal septum. 

 Besides, other smaller cavities appear in the walls separating 

 the different cells. 



When more deeply worn down, or under certain conditions 

 not clearly understood, the cells (figs. 7 c & Id) appear in 

 the form of rounded or oval apertures, arranged in diagonal 

 rows, but separated by a vast number of small rounded 

 foramina, which appear to be the mouths of interstitial tubuli. 

 In this condition the fossil presents much the appearance of 

 certain species of Chwtetes (Monficnhpora). 



The best examples of this singular Polyzoon that I have 

 seen, grow in the form of thin crusts, rarely exceeding one 

 fourth of a line in thickness, upon Stroplwmena alternata^ 

 Conrad, and upon various species of Chcetetes. In some ex- 

 amples it would seem that several layers of cells are super- 

 imposed on one another ; but I am not sure of the nature of 

 these specimens. Not uncommonly the cells are concentrically 

 disposed round a number of iiTegidar areolae, each of which is 

 formed by a number of cells radiating from a central point. 

 Young examples form circular crusts, with a slightly cupped 

 centre, from which the cells radiate in every direction (fig. 7 h). 

 Lastly, examples are not uncommon which appear to have the 

 form of small branching stems. Some of these certainly are 

 merely constituted by thin crusts growing upon various ramose 

 species of Chcetetes. Others, however, appear to be entirely 

 composed of the Polyzoon itself ; and it is possible that these 

 will eventually prove to be a distinct species. 



Locality and Formation. — Cincinnati Group, near Cincin- 

 nati, Ohio. Collected by Mr. U. P. James. 



EXPLANATION OF PLATE XIV. 



Fig. 1. PtUodidya falciformis, Nich., a small example, of the natural 



size. 1 a. Trausveree section of the frond, enlarged, 1 b. Small 



portion of the surface, pn-eatly enlarged. 

 Fig. 2. PtUodictija emacerata, Nich., of the natural size. 2 a. Transverse 



section of the frond, enlarged. '2 b. Portion of the surface, 



sveatlv enlarged. 



13* 



