212 BRITISH STROMATOPOROIDS. 



Vertical sections (Plate XXVII, fig. 7) show well-marked concentric lamina?, 

 of which about six occupy the space of 2 mm. measured vertically. The radial 

 pillars are stout, and are confined to the interlaminar spaces in which each 

 originates. " Interlaminar septa" are often present in fairly large numbers, and 

 have the form of delicate curved calcareous partitions intersecting the inter- 

 laminar spaces obliquely. If a vertical section traverses a mamelon (woodcut, 

 fig. 28), the axial and radial canals of the astrorhizal system belonging thereto 

 are seen to be crossed by irregular calcareous partitions or " astrorhizal tabulae." 



Specimens of 8. solitaria are commonly found in the " Caunopora-state," but 

 show no phenomena of special interest. 



Obs. — As previously pointed out, I am now satisfied that I formerly included 

 two distinct types under the name of Stromatoporella eifeliensis ; and I now 

 propose the name of 8. solitaria for those specimens which agree with the specific 

 description above given. In various respects, and particularly as regards minute 

 structure, 8. solitaria agrees with 8. eifeliensis ; while the difficulty of separating 

 the one from the other is enhanced by the fact that specimens of the two are 

 very commonly associated with one another. Nevertheless, both as regards 

 microscopic structure and macroscopic characters, there are sufficient distinctive 

 features to warrant the separation of the two forms as distinct species. 



Thus while the coenosteum of S- eifeliensis has the form of a crust para- 

 sitically attached to foreign bodies by the whole of the lower surface, that of 

 S. solitaria is laminar, is furnished with an inferior epithecal membrane, and is 

 attached by one point only. The astrorhizas of 8. eifeliensis are extraordinarily 

 developed, and anastomose freely with one another, but they are not arranged in 

 superimposed systems, and neither " astrorhizal cylinders " nor surface-eminences 

 are developed. On the other hand, the astrorhizas of 8. solitaria are compara- 

 tively small, and are circumscribed ; but they are vertically superimposed, and 

 usually form the centres of well-defined " astrorhizal cylindei's," while they open 

 on prominent surface-projections or " mamelons " (Plate XXVII, fig. 4). 



As regards minute structure, again, the skeleton-fibre of 8. solitaria is not so 

 finely tubulated as it is in 8. eifeliensis, and is at the same time much more 

 delicate. The result of this latter character is that the coenosteal network of 

 8. solitaria becomes conspicuously more lax and open than is the case in 

 S. eifeliensis (compare figs. 2 and 3 with figs. 6 and 7 of Plate XXVII). 

 Lastly, 8. solitaria shows in tangential sections the same perforated tubercles as 

 are characteristic of corresponding sections of 8. granulata or 8. Selwi/nii. 



Leaving 8. eifeliensis out of consideration, the species which 8. solitaria most 

 nearly resembles is 8. Selwynii, Nich. From this form, however, the present 

 species is separated by the comparatively greater delicacy of the skeletal frame- 

 work, the much more extensive development of the astrorhizal system, and the 



