1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 343 
surface of this the entire basal portions of two secondary fistular 
branches proceed. There are also the remains of another such 
branch at the margin of the primary fistula at the right-hand side. 
The outer surface of the sponge has an irregular reticulation of stout 
siliceous fibres, very similar to those of Dactylocalyx immediately 
beneath the dermis. 
In all the recent species of this tribe of siliceo-fibrous sponges 
with which I am acquainted, there is an expamsile dermal system 
attached to the stiff non-expansile skeleton beneath by connecting 
spicula cemented at their basal points more or less to the mass of 
the skeleton beneath by keratode only, and which would naturally 
be separated from the body of the sponge by maceration and by de- 
composition of the membranous and keratose matter a short period 
after its death; and none of the expansile dermal system, it is pro- 
bable, would appear with the fossil unless it were to be enveloped 
and fixed in the matrix after its death—a result scarcely to be ex- 
-pected. This organized envelope usually affords the most distinct 
and determinative specific characters of the sponge, and it was very 
important to discover its remains if possible; but in this attempt I 
have been unsuccessful. 
In its living condition this sponge would probably exhibit a smooth 
membranous surface ; but in its present state we have large open 
areas exhibited tm liew of the smooth dermal membrane. These 
areas are, in fact, the distal ends of the intermarginal cavities, and 
are usually much larger than the interstitial spaces immediately be- 
neath them. In the specimen under~consideration, as in similarly 
organized recent sponges, the proximal terminations of the intermar- 
ginal cavities communicate immediately with the distal ends of the 
interstitial spaces, and these uniting increase in their size as they 
progress towards the inner parietes of the great cloacal cavity of the 
sponge, into which they finally discharge their streams through the 
oscula. In this organization they closely resemble the structures in 
the recent genera Grantia, Verongia, and many of the fistular 
keratose sponges of the West-Indian seas. 
I have not detected any connecting spicula, and I have assigned 
the rectangulated hexradiate ones to the interstitial cavities on the 
faith of some very dilapidated remains of them, deeply immersed 
in the tissues, and rendered visible only by the penetrating power of 
the Lieberkiihn—and by two other fragments, one detached, repre- 
sented in Plate XXV. fig. 7, and the other im situ, in the portion of 
the skeleton figured at a, fig, 6, Plate XXV. 
The nearest relations to this tribe of sponges among the fossil 
ones are decidedly the siliceo-fibrous sponges of the Flamborough 
Chalk ; below that formation I am not aware of any such sponges 
haying ever been found. The matrix of the Australian fossil also 
possesses much of the character of chalk ; it dissolves completely in 
pe hydrochloric acid, leaving only a small quantity of sandy resi- 
uum. 
* I may also observe that the similarity of form and structure be- 
tween the Australian and the English Chalk fossil sponges in this 
