74 DR.J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, 
to the membranes they are destined to support, and which, in well- 
preserved specimens, may be seen stretching from point to point of 
the radii. Fig. 2, Plate ILI., represents a spiculum of this form 
x 108 linear. 
In some species of siliceo-fibrous sponges there is a paucity or a 
total absence of the rectangulated auxiliary fibres and of the simple 
rectangulated hexradiate spicula; in such cases we frequently find 
their places supplied by numerous long acerate interstitial spicula 
dispersed in the interstitial spaces of the rigid skeleton, their office 
appearing to be to increase the surfaces of the nutrimental mem- 
branes. In specimens in which the animal matter is well preserved, 
the membranes are seen stretching from point to point of each spi- 
culum, and from the points of one of these spicula to those of other 
similar ones in its neighbourhood ; and as these spicula occur grouped 
together frequently in considerable numbers, it may be readily con- 
ceived that they perform an important office in thus increasing the 
amount of the nutrimental surfaces within the animal. The inci- 
pient spination of the radii, so prevalent in this form of spiculum, 
admirably fits them to maintain their hold of the delicate interstitial 
membranes which are attached to them. 
The spinulo-trifurcated hexradiate stellate (Plate III. fig. 4) and 
other forms of those spicula appear to be peculiar to the siliceo- 
fibrous sponges. In the well-washed specimens they do not seem 
to be very numerous ; but in cases where the interstitial membranes 
are in a good state of preservation, they are occasionally found to be 
so abundant and so closely packed together as to completely cover 
and obscure the membrane beneath them. Occasionally the hexra- 
diate stellate forms occur with the radii attenuated and acutely ter- 
minated (Plate III. fig. 5). 
This form of spiculum is abundant in the type specimen of 
Dactylocalyx pumiceus, and is probably either an abortive or an im- 
mature development of the spinulate form of spiculum. In my exami- 
nation of the corresponding forms of spicula in Iphiteon Ingalli I 
found two of this attenuating form of spiculum which, under a 
power of 108 linear, appeared to have their radii acutely terminated ; 
but on the application of a power of 666 linear I found that their 
apices exhibited incipient spinulation ; and these spicula were the 
only two that I could find, although I searched for other specimens 
in a similar condition amidst a crowd of spinulate ones which com- 
pletely covered a fragment of membrane that I obtamed from the 
sponge while in the possession of my late friend Mr. Ingall. 
The situation and peculiarities of the oscula and pores afford im- 
portant characters in the determination of the species in all sponges. 
In the cup-shaped siliceo-fibrous sponges the oscula are situated on 
the surface of the inside of the cup, and the pores on the outer sar- 
face. ‘The same law obtains in the cup-shaped kerato-fibrous 
sponges of commerce and in numerous cup-shaped Halichondraceous 
species. In coating or amorphously massive sponges the oscula and 
pores occupy the same surface, and the incurrent and excurrent sys- 
tems of canals are intermingled. The circulation of the nutrient 
