GENUS CLATHRODICTYON. 77 



that they really run continuously for considerably greater distances. In A. clath- 

 ratum, in which the coenosteum commonly grows in " latilaminas," the pillars seem 

 certainly to extend in general through the entire thickness of a latilamina. The 

 radial pillars are mostly, perhaps always, hollow, each being traversed by a minute 

 and apparently often nearly obliterated axial canal (PL I, figs. 10 and 13). This 

 phenomenon can only be recognised in tangential sections, and only in well-pre- 

 served specimens. On the free surface of the coenosteum, the pillars terminate in 

 blunt and apparently imperforate tubercles (PI. II, fig. 11). 



Tangential sections (PL I, figs. 8, 10, 11) show the cut ends of the radial 

 pillars and the angular meshwork formed by the inosculation of the horizontal 

 connecting-processes ; the structure being of what has been called the " hexacti- 

 nellid type," from its superficial resemblance to the spicular network of some of 

 the Hexactinellid Sponges. 



So far as my observations have extended, astrorhizae are present in the majority 

 of species belonging to the genus Actinostroma, including the type-species A. 

 clathratum (=8. concentrica, Barg.) in which their existence has been denied. 

 They vary, however, greatly in their development, and they are apparently occa- 

 sionally wanting. In at least one species of the genus (namely, that which Bar- 

 gatzky has erroneously identified with Stromatopora astroites, Rosen) they are 

 largely developed, and are arranged in successive superposed groups, connected by 

 vertical wall-less canals (PL IV, fig. 3, a). 



The form of the coenosteum in the genus Actinostroma is usually massive or 

 laminar, and in the latter case an epitheca is almost always developed basally. In 

 the massive forms, however, the colony often grows in successive layers, of which 

 the first is attached to some foreign body. 



Genus Clathrodictyon, Nich. and Mur. 



(' Journ. Linn. Soc.,' vol. xiv, p. 220, 1878.) 



Coenosteum often of large size, usually expanded or laminar, with a concentri- 

 cally- wrinkled basal epitheca and a small base of attachment ; occasionally mas- 

 sive. The general structure of the skeleton is like that of Actinostroma, but the 

 radial pillars are incomplete, and are never "continuous." Astrorhizae are present. 

 The surface is minutely granular or vermiculate, without marked prominences or 

 " mamelons." 



In certain of the types which may be placed under Clathrodictyon, such as C. 

 regulare, Rosen, the general structure of the skeleton is precisely that of Actino- 

 stroma, except that the radial pillars are confined strictly to the interlaminar 

 spaces in which they take their origin, and never pass continuously through suc- 



11 



