ACTINOSTROMA STELLULATUM. 141 



granules are either isolated, or may be more or less connected into sinuous groups 

 by means of delicate connecting processes. 



Astrorhizse are invariably present, and are arranged in superposed systems, 

 each system having a common vertical axial canal (Plate XIV, fig. 2), which may 

 or may not open at the surface on a special eminence. In any case, the 

 " mamelons," when present, are comparatively low and flat, and are not prominent 

 and conical (Plate XV). The astrorhiza? are always delicate, with slender, radiat- 

 ing branches, which are sometimes few and comparatively simple, and sometimes 

 numerous and much branched. In the former case the astrorhizas are small, and 

 their centres are placed at a distance of about 5 or 6 mm. In the latter case, they 

 are comparatively large and their centres are from G to 10 mm. apart. 



As regards the internal structure of the ccenosteum, the radial pillars are 

 slender, and about six or seven occupy the space of 1 mm. The radial pillars 

 seem to be really continued through an indefinite number of interlaminar spaces. 

 Owing, however, to the fact that no individual pillar lies for more than a short 

 distance in the plane of a vertical section (Plate XIV, figs, land G), it happens that 

 the pillars only appear to run through a few interlaminar spaces before they seem 

 to terminate. If the plane of the section happens at some point to coincide accu- 

 rately with the plane of the pillars, then a single pillar may be readily traced 

 through ten or twenty successive interlaminar spaces. If, on the other hand, the 

 section be slightly oblique to the pillars, then these structures may appear to run 

 only through two or three successive interlaminar spaces, or even to be confined 

 to a single space, thus producing a likeness to the genus Chtlhrodictjioii. 



The concentric laminae are well marked, about from six to eight occupying the 

 space of 1 mm. Tangential sections (Plate XIV, figs. 3 and 5) show the 

 radiating astrorhizal canals and the cut ends of the radial pillars. In some cases 

 the radial pillars are connected with one another, more or less extensively, by 

 means of numerous delicate and hair-like connecting-processes or " arms " (Plate 

 XIV, fig. 5). In most cases, however, these connecting-processes are not visible 

 at all in tangential sections (Plate XIV, fig. 3), or they can only be very partially 

 detected. Hence such sections do not show the typical "hexactinellid" network 

 characteristic of the genus Actinostroma, but usually closely resemble corresponding 

 sections of the genus Clathrodictyon. 



Obs. — Actinostroma stellulatum, Nich., is a very well-marked and widely dis- 

 tributed species. It is most nearly related to A. hebbornense, Nich. (=Stromatopora 

 astroites, Barg.). It is, however, distinguished from this, as from all the other 

 species of Actinostroma, by the fact that tangential sectious do not usually show 

 the characteristic " hexactinellid " meshwork of the genus. On the contrary, such 

 sections resemble corresponding sections of Clathrodictyon in showing the detached 

 ends of the transversely divided radial pillars, either quite separate or partially 



