GENERA OF FAVOSITIDyE. 



173 



very short intervals by annular accretions of growth, the por- 

 tions of the tube between these retaining their normal diameter. 

 As these thickened portions of the corallites are placed at cor- 



m 



'^1 



Fig. 25. — A, Portion of a branch of Stenopora yackii, Nich. and Eth. jiin., split open, of the 

 natural size ; B, Portion of the same enlarged, showing the annulations of the tubes in their 

 outer portions ; c, A few of the tubes of the same still further enlarged, showing the 

 mural pores. Pernio- Carboniferous formation, Queensland. 



responding levels in all the corallites, it follows that the tubes 

 are actually in contact with one another at these points only, 

 and that they are separated by ring-like spaces corresponding 

 to all the unthickened segments of the tubes. 



Thin sections of the corallum bring to light the peculiarities 

 just mentioned, along with others, the true significance of which 

 cannot at present be ascertained. Thus, if we take a trans- 

 verse section of a branch, and examine its central portion, 

 where the nearly vertical tubes are cut across approximately at 

 right angles to their course, we find that the corallites (fig. 26, b) 

 differ in no essential respect from those of Favositcs as re- 

 gards their general structure. Each possesses its own wall, 

 which is not excessively or abnormally thickened, and the 

 boundary between contiguous tubes is clearly marked by a 

 dark line. The tubes In this portion of the corallum are, more- 

 over, regularly polygonal, and are, as a rule certainly, in close 

 contact. If, on the other hand, we take a section tangential to 

 the branch, and just below its surface, we have the corallites cut 

 transversely across in the horizontal and annulated portion of 

 their course, and the appearances presented are very different 

 to the above. The tubes still appear to be in close contact and 

 to be polygonal, each being bounded externally by a well- 



