342 pR.J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [ May 13, 
in considerable numbers around portions of the skeleton-fibres ; and 
it is probable that the dermal and interstitial membranes were abun- 
dantly furnished with them (Pl. XXIV. figs, 2, 3, 4). 
This remarkable sponge is in its skeleton-structures exactly like 
those of a Verongia, its siliceous nature constituting the only essen- 
tial difference. The specimen is undoubtedly by far the most valu- 
able of the two represented in the plate in the ‘Transactions of the 
Linnean Society.’ In conclusion, I must return my best thanks to 
my friend Dr. A. Farre for the repeated opportunities I have had of 
closely examining its structure. 
PurRIsiPHONIA, Bowerbank. 
Skeleton siliceo-fibrous, reticulate, unsymmetrical; fibres com- 
posed of concentric layers of solid silex, with a continuous central 
canal. 
This genus is intermediate in its structure between Dactylocalyz, 
Stutchbury, and Farrea, Bowerbank. Like the latter, its fibres are 
continuously canaliculated ; but it has not anything approaching the 
angulated symmetrical arrangement of its skeleton-fibres; on the 
contrary, it very closely simulates the mode of the distribution of 
the fibres that prevail in Dactylocalyx. The central canals in the 
fibres of the species of Purisiphonia on which the genus is founded 
occupy from about one-fifth to one-third of the entire diameter of 
the fibre; they are straight and uniform in their own diameter, and 
have little or no enlargements at their junctions with each other. 
The reticulations of the skeleton are frequently extremely close, so 
that the areas do not exceed, or sometimes even equal, the diameters 
of the fibres bounding them. 
PurisipHonta CLARKEI, Bowerbank. 
Sponge fistulous, branching ; surface of rigid skeleton even. Os- 
cula simple, dispersed over the inuer surface of the fistulae. Dermal 
structures unknown. Skeleton stout, closely reticulated. Inter- 
stitial cavities furnished with rectangulated hexradiate spicula. 
Hab. Wollumbilla, Queensland, Australia (Dr. Clarke). Fossil. 
There is much greater difficulty in the specific description of a 
fossil sponge than of a recent one, as a considerable portion of the most 
decisive specific characters are usually absent, in consequence of the 
decomposition of the softer. parts of the organization previously to 
fossilization ; and this is doubtless the case with the specimen under 
consideration ; but although thus deprived of the use of many valu- 
able descriptive characters, there are sufficient remaining to enable 
us to securely determine its specific identity. 
It is difficult to say what has been the correct form of the speci- 
men in its unmutilated state; but, judging by its present condition, 
it has originally been a large fistulous sponge, giving off fistular 
branches at irregular intervals. The large fistular body of the 
sponge has been split longitudinally, and a portion 4 inches in length, 
and of about half of the tube of the sponge, remains, and from the 
