From the Silurian Rocks of North America. 183 
Ceramopora. Worn examples, however, exhibit very different 
characters; and when the entire original surface has been 
abraded, it is sometimes difficult or impossible to determine 
whether or not one is dealing with this or some entirely dif- 
ferent form. 
When slightly worn, 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 compartments, 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¢ & 7d) 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 Chetetes (Monticulipora). 
The best examples of this smgular Polyzoon that I have 
seen, grow in the form of thin crusts, rarely exceeding one 
fourth of a line in thickness, upon Strophomena alternata, 
Conrad, and upon various species of Chetetes. 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 irregular areole, 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 5). 
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 
specics of Chetetes. 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. Ptilodictya faleiformis, Nich., a small example, of the natural 
size. 1a. Transverse section of the frond, enlarged. 16, Small 
portion of the surface, greatly enlarged. 
Fig. 2. Ptilodictya emacerata, Nich., of the natural size. 2a. Transverse 
section of the frond, enlarged. 26. Portion of the surface, 
greatly enlarged. 
13* 
