STROMATOPOROIDEA. 



231 



that the radial pillars contain in their interior a minute axial canal 

 (as in Labechici) fig. 116, a), but there is no ground for supposing 

 that this opened upon the surface by any aperture. 



The zooids which composed the colony of the Stromatoporoids 

 were sometimes merely lodged in the pores which pierce the reticu- 

 lated concentric laminae, and there are no definite " zooidal tubes " 

 (fig. 115, a). In other cases, such tubes are present ; though their re- 

 cognition is not always easy, and they are often very irregular. 

 When well developed, the zooidal tubes are usually intersected by 



; 3J5- r^'Q^R" 



Fig. 115. — A, Tangential section of Actinostroma intertextutJi, showing the transversely- di- 

 vided radial pillars and the reticulated structure of the concentric laminae, b, Vertical section of 

 the same, showing the radial pillars and the formation of the concentric laminae out of processes 

 which are given out horizontally by these. Enlarged twelve times, c and d, Parts of the same 

 sections enlarged further. From the Silurian rocks. (Original.) 



a number of transverse calcareous partitions or "tabulae" (fig. 119, 

 b and d). In no case are the zooidal tubes provided with radi- 

 ating vertical partitions comparable with the " septa " of corals. It 

 would seem certain that in all the Stromatoporoids the part of the 

 colony which would be at any given moment actually alive, must 

 have been restricted to a thin external layer. 



Among the more noticeable features presented by the Stromato- 

 poroids are the so-called " astrorhizse " of many types. These are 

 stellate, often much branched, gutters or grooves which are found on 

 the external surface of the last-formed lamina, each stellate system 

 being placed at a little distance from its fellows. As each lamina 



