RADIAL PILLARS AND CONCENTRIC LAMINA. 



43 



Most Strouiatoporoids show, in one form or another, similar openings in the 

 concentric laminas, and we can hardly doubt that they served for the passage of 

 stolons of the ccenosarc, and, in the last formed lamina, for the emission of zooids. 

 In spite of the fact that the concentric larninse are thus porous, they necessarily 

 present themselves in thin vertical sections as continuous horizontal lines, since the 

 interlacing " arms," out of which they are formed, are placed at corresponding 

 levels (Plate I figs. 9 and 12). 



(c) Variations in the Structure of the Radial Pillars and Concentric Lamina?. — The 

 above is, in brief, the general structure of the skeleton in the two great sections of 

 Stromatoporoids represented respectively by Stromatopora, Goldf ., and Actinostroma, 

 Nich. There are, however, numerous more or less striking deviations from this 

 type which require consideration. Most of these will be best discussed in connec- 

 tion with the descriptions of the genera and species. It will be sufficient, therefore, 

 here merely to deal briefly with certain points of special structural importance. 



Fig. 2. 



Fig. 3. 



Fig. 2. — A. Vertical section of Clathrodictyon 

 cellulosum, Nich. and Mur., enlarged twelve 

 times. B. Tangential section of the same, 

 similarly enlarged. Corniferous Limestone, 

 Waiufleet, Ontario. 



FlO. 3.— A. Tangential section of Clathro- 

 dictyon fastigiatum, n. sp., enlarged twelve 

 times. B. Vertical section of the same, 

 similarly enlarged. Wenlock Limestone, 

 Dormingtou. 



