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Show 80 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jail. 28, ment to the primary skeleton-fibres and by their habit of inosculation. Beside the auxiliary fibres, there are in some parts of the sponge an abundance of true rectangular hexradiate spicula (Plate III. fig. 2) ; but they are rarely found mixed with the auxiliary fibres or in the same spaces with them. Although occurring in closely packed groups, they never unite with each other, nor are they even attached to any parts of the surrounding skeleton-fibre, and they always preserve their normal form. They are slender, smooth, and their radii are very slightly inclined to become clavate. The termination of the elongated basal portion of the spiculum is frequently incipiently spinous. Their length is -Jj inch, the expansion of the lateral radii y^ inch, and the diameter of the axial shaft varies from -g-chj-^ to T"j*r*Wrr Inch. The trifurcated attenuato-hexradiate stellate (Plate III. fig. 5) and the trifurcated spinulo-hexradiate stellate spicula (Plate III. fig. 4) are both very abundant, and in some small masses of sarcode they are so numerous and so closely packed together as to render it quite impossible to count them. The sarcode appears to have been very abundant, as in some parts it completely fills up the reticulations of the skeleton ; it is of a full amber-yellow colour. Thus far we have positive characters by which to discriminate this beautiful species of sponge from its nearly allied congeners ; but I have been fortunate in finding other characters, which, from the mode in which they have been obtained, although not so decisive in their nature, are yet of such importance that their description cannot be omitted in treating of this species. I carefully examined the half of the type specimen of D. pumiceus that is in the British Museum in the hope of finding a small fragment of the dermal portion of the sponge, but I did not succeed in detecting any remains of it on the cup-shaped portion of the specimen ; but ou the basal surface of the pedicel there were remains of what appears to have been the basal membrane. It consists of a dense yellow incrustation, closely intermingled with the basal skeleton-structure, and agreeing in colour and appearance with a few very minute specks of the animal matter of the external surface of the sponge. I mounted a small portion of this basal matter in Canada balsam ; but this material did not render the fragments transparent; yet there were at some portions of their margins unmistakable evidences of their containing spicula. There were also fragments of the skeleton-structure of the base of the sponge, the reticulations of which were, as might be expected from their situation, very close and dense (Plate III. figs. 14 and 15); and along with these fragments there was a group of three large and very remarkable verticillately spined cylindrical spicula, very closely resembling in their structure the one represented by fig. 69, plate 3, vol. i.,' Monograph of British Sponges,' and also by fig. 23, plate 36, Phil. Trans, for 1862, but differing from those figures in being much longer in their proportions, and in having a greater number of circles of spines (Plate III. fig. 6). Having seen thus much of the dermal |
