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Show 82 DR. 3. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, forming no part of the economy of the basal membrane, although usually abounding in the dermal one; and their presence may be accounted for by the fact that the material operated on was principally taken from the margin of the base of the sponge, at the junction of the membrane of the pedicel with that of the base. Imbedded in the remains of the membranous structures operated upon by nitric acid, there were a few very minute attenuato-stellate spicula ; and I subsequently found at the margin of a fragment of the basal portion of the sponge, mounted in Canada balsam in its natural condition, several dichotomo-patento-ternate connecting spicula in situ, and along with them a crowd of the minute attenuato-stellate ones. They were so numerous as to entirely obscure the small portion of dermal membrane on which they reposed. The regular mode of their disposition on the membrane, and the contact of the latter with the expanded outer surface of the heads of the dichotomo-patento-ternate connecting spicula, unmistakably indicates their especial office and true position in the sponge as retentive and defensive spicula of the dermal membrane. I measured several of these minute spicula. The largest was t •,1 g7 inch from the opposite points of their radii; the smallest was YTTTS m ch m extreme diameter; but by far the greater number were about 2 ^ A inch in diameter, and the largest measurement was of rare occurrence. Occasionally, but very rarely, the radii were cylindrical, instead of attenuating to an acute point. From the indications afforded by the spicula I have described above we may reasonably predict that, when a specimen of D. pumiceus shall have been found in a living state and perfectly preserved, we shall find it to be furnished with a beautiful expansile dermal system similar in character to those of the siliceo-fibrous sponges which are well known to us in a perfect state of preservation, such as D. Prattii and D. Masoni. But our evidence regarding the structure of the sponge is not yet exhausted ; for by a careful examination of a series of minute fragments which I subsequently obtained from the margin of the base of the type specimen in the British Museum and mounted in Canada balsam in their natural conditions, I strengthened the evidence obtained from the spicula operated on by nitric acid. In several cases these spicula were seen imbedded together in the same membrane at the transparent edges of the fragments under examination. In one such case the membrane was thickly studded with the minute attenuato-stellate spicula, and amidst them was imbedded one of the subequiangular triradiate and several of the small acerate tension-spicula; from the edges of another fragment the ternate heads of two connecting spicula, covered by dermal membrane containing innumerable minute attenuato-stellate spicula, were projected, thus confirming the inferences raised by the spicula arising from the dissolution of the fragments in nitric acid. The evidence derived from the dissolution of portions of the basal part of the sponge in nitric acid might reasonably be questioned; but when we are able to confirm it by detecting the spicula separated |