Reviews — Prof. Dr. Nicholson — British Sfromatoporoids. 125 



are united at irregular intervals by oblique or horizontal processes. 

 Between the vertical rods are the tortuous, tubular, and often tabulate, 

 canals, in which the zooids were lodged. In other cases, the skeleton 

 is made up of numerous closely-approximated concentric laminae, not 

 in absolute contact, but separated by interlaminar spaces, which are 

 intersected by vertical columns or " radial pillars," which connect 

 the laminae, and may pass through several laminae and interspaces. 

 The calcareous fibres of the skeleton are probably in no case compact, 

 sometimes they are filled with what appear to be minute pores or 

 vesicles, at others by complex ramifying tubuli. Nothing of the 

 nature of definite spicules has been noticed in any example of the 

 group. 



According to the general arrangement of the " radial pillars " and 

 "concentric laminae" two principal sections of Stromatoporoids are 

 distinguished, the Millepoi'oid and Hydractinioid sections. In the 

 first of these, represented by the genus Stromatopora, Goldfuss, the 

 radial pillars and concentric laminae are completely amalgamated, 

 and, as a rule, cannot be recognized as distinct structures. Tan- 

 gential sections of the skeleton appear to be continuously reticulate, 

 resembling that of Millepora, whilst in vertical sections the radial 

 pillars appear as thick, irregular and flexuous, and the concentric 

 laminae are only represented by irregular lateral outgrowths, which 

 spring from the pillars and unite them into a continuous framework. 

 In the " Hydractinioid" section, represented by the genus A ctino stroma, 

 the radial pillars and concentric layers are present as distinct, though 

 closely connected structures. In vertical sections of A. claihratum, 

 the skeleton appears as a series of parallel vertical rods, nearly 

 equi-distant from each other and connected at regular intervals by a 

 series of parallel horizontal laminae. In a tangential or horizontal 

 section, the transversely-divided ends of the radial pillars appear as 

 rounded or stellate dots, placed at tolerably regular intervals. The 

 radial pillars give out at regular intervals, verticils of horizontal 

 connecting processes or arms, which join one another to form a com- 

 plete network, and the concentric laminae are, in reality, formed of 

 these united horizontal processes. When the section passes through 

 the plane of one of the concentric lamina, the cut ends of the radiate 

 pillars have a stellate form, and their united arms form an angular 

 mesh-work, not unlike that of a hexactinellid sponge, but if the 

 section is through the interlaminar spaces, then only the cut ends of 

 the radial pillars are visible. Thus the concentric lamina is, in 

 reality, merely a porous mesh-work, through the openings of which 

 in the upper layer the zooids could extend. 



In the genus Clatlirodictyon, Nich. and Murie, the radial pillars 

 are incomplete or almost obsolete, whilst in Labechia they are highly 

 developed. The author states that the radial pillars are not invari- 

 ably solid, a minute central canal can be detected in many, but even 

 where this exists, it does not appear to have been open at the sum- 

 mit. There is no ground for the supposition that the pillars were 

 inhabited by zooids, or that they can be compared in any way with 

 the zobidal tubes of Millepora. In one singular form, Her mato stroma 



