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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 73 tion, each ray being so adapted to the structure of its neighbour as to render its eccentricity of form, when separate, no longer apparent when in combination (Plate V. fig. 8 ). The apices of the connecting spicula are exceedingly various in their forms, but they are all modifications of the triradiate one, even in the peltate forms ; the triradiate canals passing from the distal termination of the central canal of the shaft at once indicate the connexion with the normal structure, as represented Plate V. figs. 9, 10, 11. The general mass of the fibro-siliceous skeleton in the genera Dactylocalyx and Iphiteon varies considerably in the different species. In some it is quite smooth, in others tuberculated or spinous ; but it is constant in its characters in each separate species; and besides its generic value, it very frequently affords valuable specific characters. Amidst the tissues of these sponges we find a secondary series of skeleton-fibres which are auxiliary to the primary ones, from which they differ in form and character to a very considerable extent. In the young condition they assume very much the aspect of the rectangulated hexradiate spicula ; but they differ from the latter in always being based upon the skeleton-structure. In their progressive development they also unite readily with other fibres of a like description with which they may come in contact, a habit never assumed by true spicula of a similar form; and if in the course of their projection they do not meet with other similar fibres, they occasionally produce a second crop of rectangulating radii, a habit which has never yet been observed to occur in rectangulated hexradiate spicula ; and although the latter are frequently intermixed with the auxiliary fibres, the spicula and the fibres are always distinctly separate from each other. The especial office of the auxiliary fibres is evidently that of affording support to the interstitial membranes: they are rarely found in the compact portions of the rigid skeleton ; but wherever there has been a large vacant space in those structures, there we find them projected into the space, anastomosing freely with each other, supporting thin films of interstitial membrane, and ultimately filling up spaces in the skeleton with solid fibrous structure, as represented in the large interstitial spaces (Plate III. fig. 1, a, a, a). Auxiliary fibres are frequently found in the interstitial spaces of keratose sponges ; but in this class they always assume the character of the common skeleton-structure of the sponge in which they occur, the only difference being that they are very m u ch more slender than the surrounding skeleton-structures. The simple rectangulated hexradiate spicula occur, either singly or in fasciculi, in some species of siliceo-fibrous sponges in considerable numbers ; in others they are of rare occurrence, or entirely absent. Their office is evidently purely that of affording support and extension to the interstitial membranes. They never anastomose with each other, or unite with any portion of the rigid skeleton. They are generally very slender, and when loosely fasciculated they accord in position. Their radii are frequently incipiently spinous at their apices, apparently for the purpose of affording a secure attachment |