1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 327 
pound structure were incipiently spinous, but the internal ones were 
smooth. The portion of the compound fibre examined measured +}; 
inch in diameter, and is represented by fig. 2, Plate XXII., x 108 
linear. 
The structure of the skeleton of Aphrocallistes beatrix, Gray, is 
precisely in accordance with that of Iphiteon panicea; and if agree- 
ment in organic structure be an evidence of close alliance, the two 
must belong to the same genus, however different their external 
forms maybe. The same description of symmetrical confluent areas 
of siliceo-fibrous structure forms the skeleton, the only difference 
being that the areas are rather less in their average diameter than 
those of I. panicea. In the latter species they average ;4, inch, 
while in the former they are 7; inch; but in their general structural 
aspect they so closely resemble each other that, if it were not for 
the spinous umbonate centres of the areas in J. beatriz, they could 
not be distinguished when examined beneath the microscope. Fig. 2, 
Plate XXI. represents a section at right angles to the surface of the 
sponge. The view of the surface of the sponge does not exhibit 
distinctly the peculiar rotulate structure of the areas ; and it is only 
when we obtain a section at right angles to the surface that this 
strikingly characteristic structure is to be seen in all its symmetry 
and beauty. But the surface view exhibits many of the specific 
characters in an extremely striking and beautiful manner. Here 
we observe large inhalant spaces, abounding in rectangulated hexra- 
diate spicula, for the support and multiplication of the nutrient 
membranes of the sponge ; and that the delicate tissues may be pre- 
served from the ravages of minute annelids and other insidious ene- 
mies, the mouths of the apertures are abundantly defended by the 
projection into them of large elongate cones of fibre, profusely fur- 
nished with minute spines ; and deeply imbedded amidst the skele- 
ton-fibre we find an abundant supply of acerate tension-spicula, and 
of the short, acerate, verticillately spined retentive ones, and occa- 
sionally groups of two or three of the porrecto-spinulo-quaternate 
spicula with attenuating shafts (fig. 3, Plate XXI., x 108 linear). 
The surface of the skeleton is furnished with a profusion of attenu- 
ated acerate external defensive spicula, the distal portions of which 
are abundantly spinous, the spines appearing as if notched upward 
out of the shaft of the spiculum, their acute points being all directed 
downward. The greater portion of these defensive organs are deeply 
immersed in the skeleton-mass beneath, their distal ends projecting 
not more than about one-fifth or one-sixth of their length beyond 
the general surface of the sponge. A section at right angles to the 
mass of the skeleton is necessary to exhibit distinctly their structure 
and position in the sponge. The auxiliary rectangulated hexradiate 
fibres of the skeleton are produced very sparingly in this species ; 
they do not attain the full development of the shaft and lateral ra- 
diations as in Dactylocalyz, the lower half of the shaft only being 
produced ; and this portion of it is abundantly spinous, and termi- 
nates hemispherically. In one portion of the skeleton, mounted in 
Canada balsam, their purpose in the economy of the animal is dis- 
