MINUTE STTIT7CTTJRE OF STBOMATOPORA AND ITS ALLIES. 203 



lated, and it is impossible to make a section wholly in the plane 

 of any single lamina or interspace. 



In studying the minute structure of the Stromatoporoids, we 

 have successively to consider the following points : — 



1. The Internal Constitution of tJie Mass. — If we make a ver- 

 tical or radial section of any typical Stromatoporoid, such as 

 Stromatopora concentrica, Groldf., we find that it is composed of 

 curved, undulating, but essentially horizontal " laminae," either 

 spread out in mass or arranged concentrically round one or more 

 centres, and having wider or narrower interspaces between them. 

 The latter are crossed by vertical or " radial " pillars, placed at 

 approximately similar distances, and dividing the " interlaminar 

 spaces " into a number of minute compartments. A horizontal 

 or tangential section of a similar form shows the transversely 

 divided ends of the ''radial" pillars, along with winding or reti- 

 culated lines representing the obliquely cut edges of successive 

 undulating laminae belonging to the horizontal series. In their 

 ultimate constitution both the horizontal laminae and the " radial " 

 pillars are composed of minutely granular carbonate of lime, 

 which shows no structure beyond the occasional presence of an 

 indistinct reticulation, and which is entirely unaffected by pola- 

 rized light. In sections of calcareous specimens, when of great 

 thinness, the laminae become more or less indistinct as a rule, and 

 it is only in certain cases that they show perfectly distinct and 

 defined outlines and do not exhibit blurred edges *. In some 

 silicified specimens, on the other hand, the skeleton is brought 

 out with great vividness between crossed Nicols, since it remains 

 unaffected itself, whilst the filling of the spaces of the fossil ex- 

 hibits a brilliant play of colours. Even in these cases, however, 

 the actual laminae and pillars are often invested by an irregular 

 crystalline layer or coating of carbonate of lime, which remains 

 uncoloured. 



2. The Nature oftheHadialJPillars. — The vertical or radial pillars 

 of a typical Stromatoporoid have most generally been regarded as 

 tuhes. So far, however, as all ordinary examples go, we can en- 

 tirely confirm Yon E-osen's observations (' Ueber die Natur der 

 Stromatoporen,' 1867), which clearly show that these pillars are 



* For this reason, in dealing with the calcareous Stromatoporoids (as apart 

 from sihcified specimens), the best results are obtained when the section has not 

 been cut down to an excessive degree of tenuity, and when a low magnifying - 

 power (a 2 to 4-inch objective) is used. 



LINN. .70URN. — ZOOLOGY, VOL. XIV. 15 



