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Show 1874.] DR. J. S. BOWERBANK ON THE SPONGIAD^. 303 into the mineral kingdom, and partly to the mode in which their remains are occasionally preserved." With our present knowledge of the protean nature of the Spongiadae, the great variety of forms exhibited by these fossils may be naturally and more readily accounted for than by imagining them to have possessed the power of contraction and dilatation ; and the enveloping flint, which affords no indication of the form or structure of the enclosed sponge, is now known to be attributable to the ventriculite sponge having, in its living state, been covered by a parasitical species of sponge, the whole being subsequently fossilized as one body. Specimens of Mantell's Fentriculites radiatus embedded in flint are by no means uncommon. If one of these exhibiting the natural surface of either the internal or external surface be immersed in a basin with water containing 10 or 12 per cent, of hydrochloric acid until the whole of the calcareous matter has been removed, and then, when dried, if it be examined by direct light with a linear power of about 50, the silicified fibrous structure will be frequently found in a beautiful state of preservation, and when compared with that of the recent sponge Halispongia ventriculoides, the fibrous tissues of the two are so much alike as almost to induce a belief that they belong to the same species under different circumstances. I have two thin sections at right angles to the natural surfaces of a specimen of Mantell's Fentriculites radiatus from near the bottom of the cup embedded in flint; viewed by transmitted light with a power of 50 linear, they present precisely the structural characters we see in our recent Halispongia ventriculoides. There is the same arrangement of the fibrous structure, the primary ones abundantly arenulous ; and the internal secondary ones are destitute of sand, and, what is strongly indicative of a close alliance, there are several fibres projecting from the external surface terminated by a single grain of sand as in the recent sponges; and the external surface of the fossil sponge is as abundantly arenulous as that of the recent one. In Ellis and Solander's work there are neither generic nor specific descriptions given of the sponges figured ; and Esper copies Ellis and Solander's figures and designates the specimen as Spongia otahitica. I do not see that this specific designation has any pretensions to stand ; and I have therefore named the species ventriculoides, as more consistent with its ancient alliances, and as forming a bond of union with the very closely allied species of Mantell's Fentriculites radiatus. HALISPONGIA MANTELLI, Bowerbank. (PlateXLVII.figs.3&4.) Sponge cup-shaped, thin; pedicel short. Surface, outer one smooth and even; inner one furnished with depressed ridges, radiating from the bottom of the cup to the distal margin; distal margin attenuated. Dermis retiform, abundantly arenulous. Oscula simple, dispersed, very minute, inconspicuous. Pores dispersed, visible by the aid of a 2-inch lens. Skeleton-primary fibres abundantly arenulous ; secondary fibres rarely arenulous. Colour light ochreous yellow. |
