220 BRITISH STROMATOPOROIDS. 



As regards the internal structure, the skeleton-fibre is of moderate thickness, 

 and is minutely porous, the pores being often unrecognisable, or being infiltrated 

 with some foreign material, and appearing therefore as so many dark dots (Plate 

 XXVIII, fig. 10). Tangential sections (Plate XXVIII, figs. 5, 7, and 8) show 

 the cut ends of the radial pillars, sometimes more or less distinct, sometimes 

 completely confluent, according as the plane of the section corresponds with an 

 interlaminar space or intersects a concentric lamina. Definite zooidal tubes 

 cannot be clearly distinguished, and are represented essentially by mere perfora- 

 tions in the concentric laminae. Tangential sections further show the transversely 

 divided astrorhizal cylinders (Plate XXVIII, figs. 4 and 5). 



Vertical sections (Plate XXVIII, figs. 6, 9, and 11) show well-marked, often 

 undulated, concentric laminae, and very distinct radial pillars of the " continuous 

 type" — i.e. pillars which are continued for longer or shorter distances across 

 successive interlaminar spaces. About six concentric laminae occupy a space of 

 2 mm. measured vertically, and about six or seven radial pillars occupy the same 

 space measured transversely. 



In well-preserved examples both vertical and tangential sections show that 

 the radial pillars are occupied by large axial canals, which send off delicate 

 prolongations into the periodically produced horizontal connecting processes, 

 which unite adjoining pillars at corresponding levels, and thus give origin to the 

 " concentric laminae." Both the vertical and the horizontal canals are usually 

 infiltrated with some dark and opaque material, and in this condition they are 

 readily recognised in both vertical and tangential sections (Plate XXVIII, figs. 

 6, 8, and 11). In badly preserved specimens the canals of the skeleton-fibre 

 may be imperfectly infiltrated, and may therefore be recognisable with difficulty 

 (Plate XXVIII, figs. 7 and 9). In some cases the canals cannot be clearly made 

 out at all. 



Vertical sections, lastly, often show that the interlaminar spaces are traversed 

 by a larger or smaller number of curved or straight calcareous partitions, or 

 " interlaminar septa," which are commonly more or less horizontal in direction, 

 and thus run parallel with the concentric laminae (Plate XXVIII, fig. 9). 



Specimens occasionally, but not very commonly, present themselves in the 

 " Caunopora-state." 



Obs. — Hermatostroma episcopale is a common Stromatoporoid in parts of the 

 Devonian rocks of Britain, and the reference of the species to Hermatostroma, 

 rather than to Stromatopora or Stromatoporella, appears to be justified by its 

 minute structure. It agrees, namely, with the first of these genera, and differs 

 from the two latter in its possession of " continuous " radial pillars, and also in 

 the fact that these structures are furnished with well-marked axial canals. The 

 specimens known to me are, however, as a rule, so imperfectly preserved, that, 



