AOTINOSTROMA VERRUCOSUM. 135 



The eriMiosf cuiii in this species is massive, spheroidal, or hemispherical, often 

 attaining a considerable size, and having a broad base of attachment, the under 

 surface being seemingly non-epithecate. The coenosteum is composed of undulated 

 and flexuous concentric laminae, which are bentrounda largely-developed series of 

 " astrorhizal cylinders," and (ill up the interspaces between these (Plate XVI, 

 fig. 1). Each astrorhizal system consists of a vertical, wall-less, axial canal, 

 enclosed in a sheath formed by a variable number (three to five or more) of con- 

 centrically disposed lamina', the innermost of these being often loosely reticulate 



(Plate XVI, fig. 5). To the structure thus formed, tl ame of "astrorhizal 



cylinder" may be given. In its course through the coenosteum, the axial canal of 

 the astrorhizal cylinder gives off irregular and feebly developed radiating canals 

 at different heights, and it ultimately terminates by an aperture on the surface, 

 which is placed at the summit of a prominent conical "mamelon" (Plate XVI, 

 fig. 8). 



The astrorhizal cylinders radiate outwards from the base of the coenosteum, 

 and the surface therefore exhibits a number of pointed conical eminences (Plate 

 XVI, fig. 4), which may be 5 to 10 mm. in width at their base, and are often 4 or 

 5 mm. in height. Each of these eminences or " mainelons " represents the free 

 upper end of an astrorhizal cylinder. Tangential sections of the coenosteum (Plate 

 XVI, fig. 1) exhibit the transversely divided cylinders, and the llexuous concentric 

 lamina? of the general skeleton filling up the interspaces between these. The 

 cylinders are generally from 5 to 10 mm. in diameter, ami are mostly placed about 

 their own width apart. In well-preserved specimens, the whole surface further 

 exhibits numerous minute rounded tubercles (Plate XVI, fig. 8), which represent 

 the upper ends of the. radial pillars. 



As regards the minute structure of the camosteum, this species does not essen- 

 tially differ from A. clathratvm, the skeletal tissue consisting of stout radial pillars 

 of the "continuous" type, crossed by strong concentric lamina) (Plate XVI, figs. 

 3 and G). As there are no " latilaminae," the pillars appear to run without a break 

 for indefinite distances. There are usually about four pillars and five concentric 

 lamina; in the space of 1 mm. The pillars are approximately equal in size, and 

 often show a minute axial canal (Plate XVI, fig. 7). As seen in cross-sections 

 the pillars are connected by numerous radiating "arms" (Plate XVI, figs. '2 and 5), 

 which give rise to a hexactinellid network, and enclose angular zooidal pores. 



Obs.— It is unnecessary to enter into any discussion as to the differential cha- 

 racters of . I. verrucosum, Goldf. ; since, so far as I am aware, it is the only species of 

 the genus Actmostroma in which the coenosteum is built up of astrorhizal cylinders 

 tilled in by undulated, concentrically laminated tissue. A precisely similar general 

 structure is shown by at least one species of Olathrodictyon (viz. G. retiforme, Nich. 

 and Mur.), but in thiscasethe skeletal tissue is of a differenl generic type. .1. verm- 



