RADIAL PILLARS AND CONCENTRIC LAMINA. 41 



of S. Beuthii, Barg., the cut ends of the radial pillars can be recognised in the 

 interior of the general reticulation (Plate V, fig. 12), and their existence can also 

 be made out in vertical sections (Plate V, fig. 13). This fact — and there are other 

 similar ones in other species — show that the striking dissimilarity between the true 

 Stromatoporce and the Actlnostromata is more apparent than real, and that similar 

 structural elements are really present in both. 



On the other hand, in the " Hydractinioid " section of the Stromatoporoids, 

 represented by forms such as Actinostroma clatliratum, Nich. (the Stromatopora 

 concentrica of authors), the radial pillars and concentric laminae are present as 

 distinct, though closely connected structures. Thus, in vertical sections of A. 

 clatliratum (Plate I, figs. 9 and 12) we observe a series of longer or shorter 

 parallel vertical rods, placed at tolerably equal distances, and connected at regular 

 intervals by a series of parallel horizontal lamina?. The vertical rods, or " radial 

 pillars," appear to vary much in length, but this is really due to the fact that the 

 section never passes along the plane of any one rod for more than a very limited 

 distance. In reality, the radial pillars are in this species continuous for very 

 considerable distances, running persistently through twenty or thirty, or more, 

 successive lamina? and interlaminar spaces. Indeed, as this species is one which 

 grows with " latilaminse," it is probable that most of the radial pillars run continu- 

 ously from the lower surface of a latilamina to the upper surface of the same. If 

 the section under examination be at all oblique, or inclined to the axes of the radial 

 pillars, then the pillars appear to run only from one lamina to the next, instead of 

 showing their true " continuous " character. 



If we next look at a tangential, or horizontal, section of Actinostroma clatliratum 

 (Plate I, figs. 8 and 11), we necessarily see the transversely-divided ends of the 

 radial pillars, in the form of either rounded or stellate dots, placed at tolerably 

 regular intervals. The precise form in which the cut ends of the radial pillars 

 present themselves depends upon the precise level at which they happen to have 

 been divided in the section examined. The radial pillars, in fact, give out at 

 regular intervals verticils of horizontal connecting-processes or " arms," which 

 join with one another to form a more or less complete network, as they are given 

 out at successive corresponding levels by all the pillars. Each successive 

 " concentric lamina " is thus formed by the fusion of the ends of the connecting- 

 processes or "arms " of the radial pillars at a given level. Hence, if the line of 

 the section passes along the plane of one of the concentric lamina?, then the cut 

 ends of the radial pillars have a stellate form (Plate I, fig. 10) ; the " arms " forming 

 by their union an angular meshwork not unlike the skeletal framework of a 

 " hexactinellid " sponge. If, on the other hand, the line of the section should 

 correspond with one of the interlaminar spaces, then the cut ends of the radial 

 pillars appear to be simply rounded or oval (Plate 1, fig. 13). Owing to the undu- 



