MINUTE STRUCTURE OF STROMATOPORA AND ITS ALLIES. 209 



placed at tlieir angles, these latter being the vacant spaces left 

 by the dissolution and removal of the radial pillars. 



5. Radiated Water-canals. — In a large number of Stromatopo- 

 roids the entire mass is perforated at intervals by numerous sys- 

 tems of horizontal or slightly oblique water-canals, as has been 

 already alluded to. These canals usually radiate from a large 

 number of independent centres, each of which is elevated above 

 the surface, in many iastauces, as a conical eminence of greater 

 or less height. In weathered specimens in which the surface is 

 shown, or in horizontal section, these canals are often admirably 

 displayed, and they are seen to bifurcate frequently as they diverge 

 from their respective centres. "We have not been able to show 

 that they start in a median canal except in some cases ; and 

 though their course is approximately parallel with the concentric 

 layers of the mass, they are usually slightly oblique, and therefore 

 pass from one interlaminar space to another, perforating the 

 laminae themselves in an oblique manner. They are not bounded 

 by definite walls, and they can hardly be regarded in any proper 

 sense as being canals which run in the actual skeleton. These 



Fig. 3. 



Stromato^ora Hindei. 

 a. Upper surface of a portion, natural size. h. Vertical section of same, showing 

 internal structure, enlarged, c. Enlarged view of a piece of the upper surface. 

 From the Niagara Limestone. (After Isicholr:on.) 



