CAMBRO-SILURIAN MXCllO-rAL/EONTOLOaY. 21 



SrATIOrORA AREOLATA. (N". Sp.) 

 Plato v., figs. 1,-1 i. 



Zoarium parasitic (?) forming very thin, circular, flattonod expansions, 

 from which proceed projecting cylindrical processes, which become 

 thickened gradually from their centre towards the proximal and distal 

 extremities. The zoarium varies in size from one to two centimetres in 

 diameter, and from 'TS to 1 mm. in thickness. The longest of the project- 

 ing processes attains a length of one centimetre, and varies in thickness 

 from 1 to 25 mm. The upper surface of the zoarium is covered with 

 low, somewhat inconspicuous monticules, upon which the cells appear to 

 bo smaller than they are on the general surface of the zoarium. The 

 under surface, which is the one most frequently mot with, is marked by 

 very conspicuous, hexagonal, shallowly concave arca^, giving to this 

 aspect of the fossil very much the appearance of Comarocystltcs puncialus, 

 Billings, for weathered examples of which it has often been mistaken. 

 The projecting processes appear to originate from the margin of the 

 zoarium, at short intervals, though there are obscure indications of their 

 attachment to other parts of the disc. The cells, which arc seen on both 

 surfaces of the zoarium, are of two kinds ; of the larger ones there are 

 about three in the space of 1 mm. 



Tangential sections show that the cells of both kinds are thin-walled ; 

 that the larger ones are pol^^gonal in outline, and that the small 

 interstitial cells fill up the spaces between them. Spiniform tubuli are 

 developed at the angles of junction of the cell- walls. 



Longitudinal sections exhibit rather remote, straight diaphragms in 

 the larger tubes, and these are much more numerous and close set in the 

 smaller ones. 



Mr. Ulrich, of Cincinnati, has suggested to the writer that the concave 

 areas of the under surface of this species may bo accounted for by suppos- 

 ing that it ''grew parasitically upon a foreign body, marked with poly- 

 gonal convex spaces, which, during the process of fossilization, was des- 

 troyed." He adds in another place : " as the markings of a Fasceolus (e. g. 

 P. globosus, Billings), would just fit the impressions [in the Spatiopora,] 

 I naturally came to the conclusion that the zoarium was attached to such 

 an object." Mr. Uliich further remarks: "In our rocks [those of the 

 Cincinnati group, at Cincinnati], there are several species, which having 

 decayed, are as yet known only by their impressions in parasitic bryozoa, 

 and in most cases where parasitic bryozoa are found in our rocks the 



