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Show 330 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [May 13, with minute spiculated biternate retentive spicula, and a few single ones were entangled in the adjoining interstices of the skeleton. As the colour of this small portion of the membrane was the same as that of minute portions of sarcode dispersed amidst the reticulations of the skeleton, there can be no reasonable doubt of its really belonging to the sponge. These spicula are so minute that they require a microscopic power of about 700 linear to define them in a satisfactory manner, and in the present case they were only visible after having been immersed in Canada balsam. A detached specimen of one of these spicula is represented by fig. 11, Plate XXII. The structure of the skeleton is stronger, larger, and more irregular than that of I. panicea or /. beatrix ; but there is no doubt of its being truly an fyhiteon. The average diameter of the skeleton-fibre is -gjft inch. The surface-fibres are very closely tuberculated, the tubercles looking very like small extraneous patches of silex adherent to the surface; and clusters of these coarse tubercles are frequently accumulated on the umbones of the confluent areas of the skeleton-structures, as represented in fig. 10, Plate XXII., which represents a portion of the surface of the rigid skeleton. The tubercles of the interior fibres are much more regular in their form, and are frequently disposed in lines, consisting of five or six of them at nearly right angles to the axis of the fibre; and a very considerable number of the fibres have no tubercles upon them. The rectangulated hexradiate auxiliary fibres were very abundant in some of the large interstitial spaces of the skeleton: when fully developed they are abundantly spinous, and the radii have spinulate terminations ; in an early stage of growth they are frequently spineless, or only incipiently spinous, and in this condition, intermixed with the stouter and more developed ones, they may be readily mistaken for spicula; but their habit of anastomosing with each other, and their basal connexion with the parent skeleton-fibre, readily distinguish them. Fig. 12, Plate XXII., represents two of the auxiliary fibres in a less complicated form than they are usually met with in the interstitial spaces of the skeleton, and exhibiting distinctly their basement on the skeleton-fibre, and their subsequent inosculation. The rectangulated hexradiate interstitial spicula are comparatively few in number; they are very slender, smooth, and their radii are clavate. The auxiliary fibres seem to have superseded them in their peculiar office of affording support to the interstitial membranes, and of multiplying the sarcodous surfaces of the interstitial spaces. The external defensive spicula of the skeleton are remarkably large and long. I have not seen an entire one ; but in a perfect condition they cannot be less than | inch in length, and the diameter of the middle of one in situ was ^oiT inch, more than twice the size of an average-sized skeleton-fibre. Their basal portions are deeply immersed in the external portion of the skeleton. The basal termination in a few cases appeared to be incipiently spinous; but this seemed to be rather the exception than the rule. |
