ACTINOSTROMA ASTROITES. 145 



the radial pillars and concentric lamina? being so fine, and so close together, as to 

 render their clear recognition with a hand-lens, even in well-preserved specimens, 

 impossible. A still more characteristic feature, which separates the present form 

 from all the other known species of the genus, is the presence of the singular 

 concentric lines of growth above spoken of. These structures can be almost 

 always clearly recognised in fractured surfaces or in thin vertical sections, even 

 in the worst preserved specimens of the species, and they may be taken as 

 diagnostic. The astrorhizce, though sufficiently well marked on fractured surfaces, 

 can with difficulty be recognised at all in their tangential sections, even where the 

 state of preservation is fairly good. 



I have examined a very large number of examples of this species from the 

 Silurian Rocks of Esthonia, Gotland, and Britain, and find the majority of 

 specimens to have undergone a kind of cystalline change, which has more or less 

 extensively obliterated their internal structure. Tangential sections of such 

 examples generally exhibit nothing more than a finely granular aspect, while 

 vertical sections show a finely reticulate structure (Plate XVII, fig. 7). Vertical 

 sections also show the concentric lines of growth, sometimes as dark lines, some- 

 times as light lines, and the closeness of these differs greatly in different examples. 

 A common phenomenon in this species, though it is one by no means peculiar to 

 it, is that the ccenosteum contains numerous embedded Spirorbes, usually arranged 

 in vertical rows (Plate XVII, fig. 7). 



In a few specimens the ccenosteum is traversed by minute vertical tubes, which 

 resemble ordinary " Caunopora-tubes " in having definite walls (Plate XVII, figs. 

 5 and 6). These tubes differ from the structures known generally as " Caunopora- 

 tubes " in not being connected together, so far as I have seen, by horizontal tubes. 

 They are also peculiar in the fact that they are very variable in point of size — in 

 the same specimen, that is to say — and they exhibit at intervals dark transverse 

 lines, which may be of the nature of " tabulae," though I do not feel clear on this 

 point. 



Another noticeable feature about A. astroites, Rosen sp., is that its vertical 

 sections, especially when in poor preservation, present a singular resemblance to 

 similar sections of certain specimens of Stromatopora typica, Rosen. Some 

 examples of this latter species exhibit a peculiar structure of the skeleton- 

 fibre — probably a sort of decomposition — in consequence of which the thick 

 and reticulated skeleton-fibre becomes broken up into innumerable minute, 

 dark-coloured, vertical and horizontal lines. This remarkable alteration of the 

 skeleton-fibre from its normal porous condition is well figured by von Rosen 

 ('Ueber die Nat. der Strom.,' pi. i, fig. 2) in a vertical section of 8. typica. So 

 close is the resemblance thus produced between vertical sections of A. astroites 

 and corresponding sections of certain specimens of 8. typica, that I was at first 



