80 SILICEOUS SPONGES. 



The steins vary from 7 to 14 mm. in thickness, and reach to a length of 110 mm. 

 The branches are from 4 to 9 mm. in thickness. 



The canal-structure is mostly obliterated; but in some specimens four or five 

 longitudinal canals, -6 mm. in width, are exposed in the broken ends of a branch. The 

 spicular mesh appears to be of the same character as in T. glabra, but the spicules 

 are somewhat larger. The dermal layer is only partially preserved ; the larger trifid 

 heads are '6 mm. in extension, and thus double the size of those in T. glabra. 



This species is also very common in Chalk flints ; the short branches, usually 

 single stem, generally rough exterior, and the larger size of the spicules of the 

 dermal layer readily distinguish it from T. glabra. 



Distribution. Upper Chalk : Chicklade, near Hindon ; Stockton, near Heytesbury ; 

 Warminster, Wiltshire ; Dover, 



Thamnospongia ■? RETicgLATA, Eiude, n. sp. (Plate XVIII. figs. 3, 3 a, 3 b.) 



Sponge consisting of a group of closely arranged, cylindrical or compressed stems, 

 growing in an upright direction, and frequently coalescing. The summits are conical. 

 The stems are from 10 to IG mm. in thickness. An imperfect example is 100 mm. 

 in height and 80 in width. 



Each stem is traversed longitudinally by a single cylindrical canal, 3-5 mm. in 

 width, into which sinuous radial canals, horizontal or slightly arched, and "5 mm. in 

 width, open. The exterior surface, when the dermal layer is absent, is covered 

 with a network of reticulated canals. At the summit of the individual stems are 

 the openings of the vertical canals, which thus appear to have served as cloacal 

 tubes. 



The spicules of the interior have been replaced by calcite, and are too indistinct 

 for the presence or otherwise of the tubercles to be determined. The arms measure 

 •135 mm. in length by -038 mm. in Avidth. Traces of the dermal layer remain ; the 

 spicules composing it are of the same character as those of T. glabra. 



In the absence of a satisfactory determination of the mesh-spicules, I can only 

 refer this form provisionally to this genus. Its mode of growth readily characterizes 

 it from the previous species. 



Distribution. Grey Chalk : near Folkestone. 



Genus PHOLIDOCLADIA, Einde, n. g. 



Sponges growing in bushy masses, consisting of cylindrical branching stems. No 

 canal-structure apparent. 



The interior skeletal mesh is composed of minute tetracladine spicules, with 

 strongly tuberculated or annulated arms, and apparently small twig-like extensions 

 at their ends, which grasp the arms of adjoining spicules without forming distinctive 



