222 BRITISH STROMATOPOROIDS. 



The coenosteuni in this type consists of rounded cylindrical stems (Plate VIII, 

 fig. 9), rooted basally, and terminating distally in rounded ends, the diameter 

 varying from \ cm. up to as much as \\ cm. 



The surface is generally more or less extensively covered with the rounded or 

 polygonal apertures of the zooidal tubes (Plate VIII, fig. 9), thus giving the fossil 

 the aspect of a dendroid Monticuliporoid, or of a species of Pachypora with small 

 corallites. In many specimens, however, the surface is not uniformly occupied 

 by the zooidal apertures, but larger or smaller areas may be covered by a thin 

 imperforate calcareous membrane (Plate VIII, fig. 12). The surface does not 

 exhibit " mamelons," nor are astrorhiza? developed. 



As regards the internal structure of the ccenosteum, the centre of each stem is 

 occupied by a main axial tube, from \ to § mm. in diameter, which is crossed by 

 more or less numerous curved or straight tabula?, and which gives off smaller 

 lateral branches, which are directed upwards and outwards. These lateral 

 branches subdivide, and may also be more or less extensively furnished with 

 tabula?. The principal axial tube seems to terminate at the end of the stem in 

 one, two, or more apertures, but the extremities of the branches are commonly in 

 a state of bad preservation, and may not exhibit any openings. 



Longitudinal sections (Plate VIII, figs. 10 and 14) show that the lateral 

 divisions of the main axial tube become connected, as they pass outwards, with 

 numerous small zooidal tubes, which are continued to the surface, and which have 

 few tabula? or none. Long sections further show, in an exceedingly marked and 

 characteristic manner, the mode of growth of the ccenosteum. Such sections, 

 namely, always exhibit a series of delicate, curved, concentric lines, the convexities 

 of which are directed towards the distal ends of the branches (Plate VIII, 

 fig. 10). These lines are due to the formation of successive conical layers of 

 ccenosteal tissue, which are much thicker over the growing ends of the branches 

 than elsewhere. Hence, as viewed in long sections, these lines are seen to 

 be comparatively wide apart in the centre of the branches, but to approximate 

 gradually to one another as they approach the surface, with which they ultimately 

 become nearly parallel. 



Transverse sections of the ccenosteum (Plate VIII, fig. 11) show the cross- 

 sections of the main axial tube and its lateral offshoots. Moreover, owing to the 

 fact that the zooidal tubes in proceeding to the surface bend outwards till they 

 become nearly rectangular to the axis of the stem, the peripheral portion of a 

 transverse section shows the zooidal tubes in longitudinal section. 



The skeletal tissue of Stachyodes verticillata is of the reticulate type, neither 

 radial pillars nor concentric lamina? being recognisable as distinct structures. 

 Sections of the skeleton, taken in any direction, show that the sclerenchyma is 

 traversed by a series of exceedingly delicate and close-set tubuli, which in the 



