POLYZOA FROM THE SILURIAN FORMATION. 265 



Genus CERAMOPORA, Hall, lSb-2. 



(Pal. N. Y., Vol. II., p. 108.) 



Ceramopora Ohioensis, Mcholson. 



Plate 25, figs. 10, lOe. 



Polyzoary incrusting, forming thin expansions attached to the surface 

 of Brachiopods or Corals, and consisting, typically at any rate, of a 

 single layer of oblique cells. Cells arranged in intersecting diagonal 

 lines, and disposed in a somewhat concentric manner round more or 

 fewer central points ; their upper walls thin and arched ; the cell-mouths 

 oblique, and, when most perfect, semicircular in shape. About eight 

 cells in the space of one line. 



Such are the appearances presented by this fossil when quite perfect, 

 and its examination in this condition leaves no doubt as to the propriety 

 of placing it in Hall's genus Ceramopora. Worn examples, however, ex- 

 hibit very different characters, and when the entire original surface has 

 been abraded it is sometimes difficult or impossible to determine 

 whether one is dealing with this or some entirely different form. 



When slightly worn, the appearances shown in fig. 106 are exhibited. 

 The delicate front wall of the cell has now disappeared, 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 in- 

 ternal septum. Other smaller cavities appear in addition in the walls 

 separating the different cells. 



When more deeply worn down, or under certain conditions not clearly 

 understood, the cells (figs. 10c and lOd) 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 Chfetetes (Monticulipora). 



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 thick- 

 ness, upon Strophomena alternata and upon various species of Chsstetes. In 

 some examples, it would seem that several layers of cells are superim- 

 posed on one another ; but I do not feel satisfied as to the real nature of 

 these specimens. Not uncommonly the cells are concentrically disposed 

 round a number of irregular areolae, each of which is formed by a num- 

 ber of cells radiating from a central point. Young examples form circu- 



