82 DR.J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, 
forming no part of the economy of the basal membrane, although 
usually abounding in the dermal one; and their presence may be 
accounted for by the fact that the material operated on was princi- 
pally taken from the margin of the base of the sporige, at the junc- 
tion of the membrane of the pedicel with that of the base. 
Imbedded in the remains of the membranous structures operated 
upon by nitric acid, there were a few very minute attenuato-stellate 
spicula ; and I subsequently found at the margin of a fragment of 
the basal portion of the sponge, mounted in Canada balsam in its 
natural condition, several dichotomo-patento-ternate connecting 
spicula in situ, and along with them a crowd of the minute attenuato- 
stellate ones. They were so numerous as to entirely obscure the 
small portion of dermal membrane on which they reposed. The 
regular mode of their disposition on the membrane, and the contact 
of the latter with the expanded outer surface of the heads of the 
dichotomo-patento-ternate connecting spicula, unmistakably indi- 
cates their especial office and true position in the sponge as reten- 
tive and defensive spicula of the dermal membrane. 
I measured several of these minute spicula. The largest was 
rais7 inch from the opposite points of their radii; the smallest was 
xzyz inch in extreme diameter ; but by far the greater number were 
about ;;1;; inch in diameter, and the largest measurement was of rare 
occurrence. Occasionally, but very rarely, the radii were cylindrical, 
instead of attenuating to an acute point. 
From the indications afforded by the spicula I have described 
above we may reasonably predict that, when a specimen of D. pumi- 
ceus shall have been found in a living state and perfectly preserved, 
we shall find it to be furnished with a beautiful expansile dermal 
system similar in character to those of the siliceo-fibrous sponges 
which are well known to us in a perfect state of preservation, such 
as D. Prattii and D. Masoni. But our evidence regarding the 
structure of the sponge is not yet exhausted; for by a careful exa- 
mination of a series of minute fragments which I subsequently ob- 
tained from the margin of the base of the type specimen in the 
British Museum and mounted in Canada balsam in their natural 
conditions, I strengthened the evidence obtained from the spicula 
operated on by nitric acid. In several cases these spicula were seen 
imbedded together in the same membrane at the transparent edges 
of the fragments under examination. In one such case the membrane 
was thickly studded with the minute attenuato-stellate spicula, and 
amidst them was imbedded one of the subequiangular triradiate and 
several of the small acerate tension-spicula; from the edges of an- 
other fragment the ternate heads of two connecting spicula, covered 
by dermal membrane containing innumerable minute attenuato-stel- 
late spicula, were projected, thus confirming the inferences raised 
by the spicula arising from the dissolution of the fragments in nitric 
acid. 
The evidence derived from the dissolution of portions of the basal 
part of the sponge in nitric acid might reasonably be questioned ; 
but when we are able to confirm it by detecting the spicula separated 
