190 BRITISH STROMATOPOROIDS. 



the species of Stromatopora, and it is not an unusual thing to find that part of a 

 thin section may have the pores simply filled with clear calcite, and therefore 

 appearing as rounded vacuities, while another portion of the same has the pores 

 infiltrated with opaque material, and appearing as dark spots. The skeleton-fibre 

 of 8. discoidea is, however, particularly prone to undergo change by mineral- 

 isation, and thin sections of altered specimens often show puzzling appearances. 

 In Plate VII, figs. 1 and 2, I have figured thin sections of one of these partially 

 decomposed specimens, in which the skeleton-fibre has assumed a minutely dotted 

 appearance, but in which the characteristic features of the coenosteum are still 

 clearly recognisable. In more extreme cases, however, the minute structure of 

 the skeleton becomes more or less extensively obliterated, and the diagnostic 

 features of the species can with difficulty be recognised in thin sections, or may be 

 wholly lost ; and it is a curious fact that this highly mineralised condition of the 

 cosnosteum is often found in specimens in which the preservation of the surface- 

 characters may be exceedingly good. It is also to be noted that the most highly 

 mineralised examples of S. discoidea yield thin sections which might readily be 

 confounded with corresponding sections of altered examples of Actinostroma 

 astroites, Rosen sp. 



On the other hand, well-preserved specimens of 8. discoidea yield thin sections 

 which are quite characteristic. Tangential sections (Plate XXIV, fig. 6) show 

 appearances closely conforming with those of the surface, the skeletal network 

 having the form of numerous irregular islands of dense coenosteal tissue, per- 

 forated by the minute openings of the transversely divided zooidal tubes, and 

 separated by branches of the astrorhizaj. Vertical sections (Plate XXIV, fig. 7) 

 are characterised by the thick radial pillars, and by the presence of well-defined 

 and abundantly tabulate zooidal tubes. 



8. discoidea often encrusts or envelops corals or other foreign bodies in the 

 course of its growth, but I have never seen a specimen in the " Caunopora-state." 

 Very commonly the coenosteum contains embedded Spirorbes, which are often 

 arranged in vertical rows, as they become successively immersed in the tissue of 

 the growing Stromatoporoid. 



From 8. typica, Rosen — its nearest ally — as from all other recorded forms of 

 Stromatopora, the present species is readily distinguished by the remarkable 

 development of the astrorhizal system and the resulting characters of the surface, 

 as also by the density and closeness of the coenosteal network. 



Having examined the original specimen of 8. elegans, Rosen, as well as thin 

 sections of the same, I am satisfied that this name is synonymous with 8. discoidea, 

 Lonsd. Von Rosen's specimen of 8. elegans is, in fact, an example of 8. discoidea, 

 Lonsd., in which the skeleton has been largely altered by mineralisation, as shown 

 in the tangential section which I have figured (Plate XXIV, fig. 8). 



