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 appa- 
rent when in combination (Plate V. fig. 8). The apices of the con- 
necting 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 Dac- 
tylocalyx 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 charac- 
ters. 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 progres- 
sive development they also unite readily with other fibres of a like 
description with which they may come in contact, a habit never as- 
sumed by true spicula of a similar form; and if in the course of 
their projection they do not meet with other similar fibres, they oc- 
casionally produce a second crop of rectangulating radii, a habit 
which has never yet been observed to occur in rectangulated hexra- 
diate spicula ; and although the latter are frequently intermixed 
with the auxiliary fibres, the spicula and the fibres are always dis- 
tinctly separate from each other. 
The especial office of the auxiliary fibres is evidently that of afford- 
ing 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, sup- 
porting 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 elass 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 much 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 exten- 
sion 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 
