MOORE AUSTRALIAN MESOZOIC GEOLOGY. 241
are continuously canaliculated; but it has not any thing approaching
the angulated symmetrical arrangement of its skeleton fibres; on the
contrary, it very closely simulates the mode of disposition of the
fibres that prevails in Dactylocalyx. The central canals in the fibres,
in 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 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.
1. Purisiphonia Clarkei, Bowerbank. Pl. XVII. fig. 1.
Sponge. Fistulose, branching, surface even, Oscula simple, dispersed over the inner parietes of the fistulae. Dermis and dermal membrane obsolete. Skeleton stout, closely reticulated. Interstitial cavities furnished with angulated sexradiate spicula?
Loc. Wollumbilla, Queensland, Australia.
Obs. 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. Although thus deprived of the use of many valuable descriptive characters, there are sufficient remaining to enable its to securely determine its specific identity.
It is difficult to say what was the real form of the specimen in its unmutilated state; but, judging by its present condition, it was originally a large fistular 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 almost half of the tube of the Sponge, remains; and from the surface of this the entire basal portions of two secondary fistular branches proceed; and there are also the remains of another such branch at the margin of the primary fistula at the right-hand side. The outer surface of the Sponge has an irregular reticulation of stout siliceous fibres, very similar to those of Dactylocalyx, immediately beneath the dermis.
In all the recent species of this tribe of siliceo-fibrous Sponges with which I am acquainted, there is a free dermal coat attached to the stiff, non-expansive skeleton beneath, by connecting spicula, cemented at their basal points to the mass of the skeleton by keratode only, and which would naturally be separated from the body of the Sponge, by maceration and decomposition of the keratode, a short period after its death; and none of this dermal coat, it is probable, would appear with, the fossil, unless it were to be enveloped and fixed in the matrix in a very short time after its death. This organized envelope usually affords the most distinct and determinative specific characters, and it was very important to discover its remains if possible; but in this attempt I have been quite unsuccessful. In its
