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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 343 surface of this the entire basal portions of two secondary fistular branches proceed. 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 a m acquainted, there is an expansile dermal system attached to the stiff non-expansile skeleton beneath by connecting spicula cemented at their basal points more or less to the mass of the skeleton beneath by keratode only, and which would naturally be separated from the body of the sponge by maceration and by decomposition of the membranous and keratose matter a short period after its death ; and none of the expansile dermal system, it is probable, would appear with the fossil unless it were to be enveloped and fixed in the matrix after its death-a result scarcely to be expected. This organized envelope usually affords the most distinct and determinative specific characters of the sponge, and it was very important to discover its remains if possible; but iu this attempt I have been unsuccessful. In its living condition this sponge would probably exhibit a smooth membranous surface; but in its present state we have large open areas exhibited in lieu of the smooth dermal membrane. These areas are, in fact, the distal ends of the intermarginal cavities, and are usually much larger than the interstitial spaces immediately beneath them. In the specimen under consideration, as in similarly organized recent sponges, the proximal terminations of the intermarginal cavities communicate immediately with the distal ends of the interstitial spaces, and these uniting increase in their size as they progress towards the inner parietes of the great cloacal cavity of the sponge, into which they finally discharge their streams through the oscula. In this organization they closely resemble the structures in the recent genera Grantia, Verongia, and many of the fistular keratose sponges of the West-Indian seas. I have not detected any connecting spicula, and I have assigned the rectangulated hexradiate ones to the interstitial cavities on the faith of some very dilapidated remains of them, deeply immersed in the tissues, and rendered visible only by the penetrating power of the Lieberkuhn-and by two other fragments, one detached, represented in Plate X X V . fig. 7, and the other in situ, in the portion of the skeleton figured at a, fig. 6, Plate X X V . The nearest relations to this tribe of sponges among the fossil ones are decidedly the siliceo-fibrous sponges of the Flamborough Chalk; below that formation I a m not aware of any such sponges having ever been found. The matrix of the Australian fossil also possesses m u c h of the character of chalk ; it dissolves completely in dilute hydrochloric acid, leaving only a small quantity of sandy residuum. I m a y also observe that the similarity of form and structure between the Australian and the English Chalk fossil sponges in this |
