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 man- 
ner as wet linen cloth would if it were supported ona 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 support- 
ing 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 mem- 
branous 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 XXII. 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 Maga- 
zine of Natural History’ for February 1868, p. 124, says, ‘‘no 
doubt these are the separate branches of a complex hexradiate spi- 
