INTERLAMINAR SPACES. 47 



of laying bare the surface of the concentric laminae, and never that of splitting them 

 into two halves, would go to prove that the laminae are not composed of two definite 

 strata. The way in which they are developed, by the fusion of the horizontal 

 connecting-processes or " arms " given out by the pillars, would still further 

 confirm this view. At the same time, the concentric laminae, when examined in 

 thin vertical sections, often show phenomena which it is not easy to fully explain. 

 In various types, the concentric laminae exhibit a central darker band, with 

 comparatively lighter-coloured calcareous tissue above and below (Plate I, fig. 1). 

 In other cases, there seems to be a definite thin line dividing the lamina into an 

 upper and lower half (Plate II, fig. 8). In various other types, such as Stromato- 

 porella granulata, Nich. (Plate I, fig. 5), or Stromatoporella eifeliensis, n. sp. (Plate 

 XI, fig. 1), the central plane of the lamina is marked by a distinct, clear, broad line 

 with darker tissue on both sides, in which minute tubuli are seen. The case of 

 those forms in which there is only a thin dark line in the centre of the lamina 

 might perhaps be explained by supposing that the laminae are at first very thin, 

 and that they gradually become thickened by the deposition of fresh calcareous 

 tissue both on their under and upper sides. In this case the dark central line 

 would represent the original lamina. It seems to me, however, to be more probable 

 that the inosculating fibres, out of which the laminae are composed, are really 

 hollow, each having an axial canal. This supposition is rendered exceedingly 

 probable by the existence of forms, such as Hermatostroma Schlitteri, in which the 

 axial canals in the radial pillars certainly send prolongations into the horizontal 

 fibres out of which the concentric laminae are made. On this view, the dark or 

 light colour of the mesial line observable in the concentric laminae of many Stromato- 

 poroids would depend on whether these supposed canals were filled with calcite or 

 with some opaque material. 



(d) The Interlaminar Spaces. — The spaces between each successive pair of laminae 

 may be spoken of by the general name of the " interlaminar spaces." In theory, 

 these spaces are continuous, but in reality they are minutely subdivided, and the 

 subdivisions are to a varying extent in free communication with one another ; while 

 in certain forms they cease to have any existence as separate structures. 



In such forms as Actinostroma clathrahbm, the interlaminar spaces are practically 

 continuous, as they are simply broken up by the passage through them of the 

 innumerable radial pillars which connect together successive laminae, as also by 

 such imperfect pillars as merely project into the interlaminar spaces from below. 

 In such cases, also, the interlaminar spaces are all placed in direct communication 

 with one another by means of the innumerable pores with which the concentric 

 laminae are perforated. In such forms, therefore, we may suppose that the whole 

 system of the interlaminar spaces was filled with the coenosarc and that the zooids 

 were given off at the surface of the last formed lamina. 



