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Show 328 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [May 13, played in a very beautiful manner. Five of them are projected at different angles in about the same plane; and as it fortunately happens that the interstitial membrane is in a beautiful state of preservation, it is seen suspended on the points of the fibres, the margin curving gently from one to the other of them, in precisely the same manner as wet linen cloth would if it were supported on a series of short props for the purpose of being dried ; and the resemblance is rendered the more complete by the doubling and folding of the membrane at the points of contact with the rough terminations of the supporting fibre ; and in the space of membrane between two of these supporting props, we have one of the rectangulated hexradiate interstitial spicula, with its almost brush-like spinous axial spiculum, imbedded in the surface of the membrane, to contribute its share of support to that portion of the structure. The attenuated hexradiate rectangulated interstitial spicula are comparatively small and delicate in their structure ; the proximal and distal portions of the axial spiculum are very nearly equal. They have usually one or both of these parts furnished with very long and slender spines, which curve in the directions of the terminations of the shaft (fig. 3, Plate XXII.). But when this form of spiculum occurs in some of the larger interstitial cavities, they are increased in size in proportion to the necessities of the situation, and two or three of them are grouped so as mutually to support each other, as well as to perform the common office of supporting the membranous structures. In this case their radii appear to be entirely destitute of spines. The slender acerate tension-spicula are few in number, and appear to abound more towards the surface of the sponge than in its deeper recesses. The acerate verticillately spinous retentive spicula are exceedingly abundant in those parts where there are any remains of the membranous and sarcodous structures. The spinous verticilli are few in number; when in a fully developed condition there are frequently as many as four of them ; but three is the more usual quantity, with perhaps a single intermediate spine to represent the fourth whorl. Sometimes they exhibit only two irregular terminal groups of spines* and a smooth shaft intervening. The spines are long and acutely conical (fig. 4, Plate XXII., x 308 linear). The porrecto-multispinulate spicula are comparatively few in number. They do not appear to be irregularly dispersed, but occur in groups of two or three together. They agree very nearly in size, but the degree of expansion of their terminal radii differs considerably; nor do all the rays on the same spiculum agree in that respect. The number of the radii at their apices appears to vary considerably; those I have observed and figured in Plate X X I I . figs. 5, 6, 7, 8, range from 3 to 6 spinulate radii. The shaft is long, slender, and attenuating to its base. Prof. Wyville Thomson, in describing this form of spiculum in his paper on Sponges in the ** Annals and Magazine of Natural History' for February 1868, p. 124, says, "no doubt these are the separate branches of a complex hexradiate spi- • |
