226 



BRITISH STROMATOPOROIDS. 



fasciculate masses of considerable size (woodcut, fig. 32). The surface is devoid 

 of " marnelons " or astrorhiza?, and is partly covered with the irregularly rounded 



Fig. 32. 



FlG. 32. — A fragment of the eoenosteum of Idiostroma oculatum, Nich., of the natural 

 size. Devonian, Biichel. a. A small portion of the surface of the same enlarged. 



or vermiculate apertures of the zooidal tubes, while other portions are covered 

 with a smooth calcareous membrane. 



As regards internal structure, the general coenosteal tissue is more or less 

 reticulate; but stout radial pillars are usually clearly recognisable in transverse 

 sections of the stems (Plate XXIX, fig. 9). Such sections also generally show 

 with extreme clearness that the skeleton is built up of definite concentric layers 

 surrounding a central core of loose reticulate tissue. The skeleton-fibre is almost 

 certainly porous (as it is in I. Boemeri, Nich.) ; but owing to the extent to which 

 all the specimens examined are mineralised, this point cannot be certainly 

 ascertained. 



Each stem typically exhibits in longitudinal section (Plate XXIX, fig. 11) a 

 well-developed axial tube, which is provided with transverse or infundibuliform 

 tabulae. This central tube gives off lateral branches, which are also tabulate, and 

 which ascend obliquely towards the surface, giving off secondary branches in 

 their course. Hence in transverse sections (Plate XXIX, fig. 9) we maj r see not 

 only the central opening representing the section of the main axial canal, but also 

 a variable number of apertures external to this, representing sections of the 

 secondary tubes above spoken of. Tangential sections (Plate XXIX, fig. 10) 

 likewise often show these secondary tubes as seen when obliquely divided. None 

 of the specimens examined show the final terminations of the axial tubes, so that 

 it is unknown whether or not they open on the surface. 



Zooidal tubes are sometimes well exhibited in the peripheral portions of 



