230 Dr. H. A. Nicholson on some new or 



structure. In vertical sections (PI. VI. fig. 7) the entire 

 skeleton is seen to be divided by well-marked concentric lines 

 which are placed at variable intervals, usually in groups of 

 close-set lines separated by wider bands in which these lines 

 are few or wanting. 



Obs. This species has given me much trouble, and I am 

 not yet sure of its affinities. I have examined the original 

 specimens collected by von Rosen, which are now preserved 

 in the museum of the University of Dorpat, and also his 

 microscopic sections of these. The original specimens are, 

 however, highly mineralized, and the thin sections show no 

 clearly recognizable structural characters beyond the con- 

 centric lines of growth which are seen in vertical sections. 

 Hence von Rosen only figured the surface of his specimens, 

 showing the well-marked astrorhiz^ ; but he gave no illus- 

 trations of the minute structure. I have also collected a 

 number of specimens from von Rosen's original locality 

 (Kaugatoma-pank) as well as from other localities in the 

 island of Oesel, which agree in every respect with the original 

 specimen upon which the species was founded. Most of these, 

 however, resemble von Rosen's originals in being so highly 

 crystallized that the essential points in their internal structure 

 are not decipherable. Out of a considerable number of speci- 

 mens which clearly belong to this species I have only found 

 two which show the internal structure in a manner suitable 

 for satisfactory study ; and I have figured tangential and 

 vertical sections of these (PI. YI. figs. 6-7 a). Judging from 

 these the species Avould seem to be an Actinostroma, with 

 extraordinarily delicate and close-set radial pillars, which are 

 united by irregular horizontal " arms." The general struc- 

 ture is therefore like that of Aciinostroma intertextum^ Nich., 

 only very much finer. In vertical sections, even in the worst 

 preserved specimens, we can recognize numerous concentric 

 lines of growth (not proper '' concentric laminae"), which are 

 usually placed in groups (PL VI. fig. 7 a) . The astrorhizaj 

 can with difficulty be recognized at all in thin sections, though 

 sufficiently well marked on fractured surfaces. 



One of the great difficulties about A. astroites, Rosen, is 

 that its vertical sections, especially when in poor preservation, 

 present a curious resemblance to similar sections of certain 

 specimens of StromatojJOixc tyinccij Rosen. Some specimens 

 of this latter species show, namely, a curious structure of the 

 skeleton-fibre, probably a sort of decomposition, inconsequence 

 of which the thick and reticulated skeleton-fibre becomes 

 broken up by innumerable, minute, dark-coloured, vertical and 

 horizontal lines. This remarkable alteration of the skeleton- 



