204 PEOr. H. A. NICHOLSON AND DE. J. MUEIE ON THE 



solid and imperforate. In no case have we certainly succeeded in 

 detecting any central cavity in the radial pillars, when their trun- 

 cated ends are examined in tangential or oblique sections. The 

 tubercles which stud the surface of many forms are sometimes seen 

 to be perforated by a central aperture ; but \Ye have never noticed 

 this appearance except upon the weathered and partially exfo- 

 liated surface of silicified specimens, and even in these many of 

 the tubercles in question are undeniably imperforate. We are 

 therefore disposed to explain this appearance as being due, not 

 to the real existence of a central canal in the radial pillars, but rather 

 to the fact that the pillars have been imperfectly silicified, and the 

 unchanged carbonate of lime in their interior has been subsequently 

 dissolved out. In general form, the radial pillars are usually 

 expanded at the points where they coalesce with the horizontal 

 laminae which they connect, and attenuated in their central por- 

 tions, and, though occasionally oblique, they mostly have a ver- 

 tical direction. In some cases they are partially rudimentary, 



Fig. 2. 



Stromatopora tuberculata. 



a. Under surface, of natural size, showing the wrinkled basement-membrane 

 and openings of water-canals, b. Portions of upper surface, enlarged, c. Ver- 

 tical section, to show internal structure, magnified. From the Corniferous 

 Limestone, Canada. (After Nicholson.) 



