330 DR.J.S, BOWERBANK ON SILICEO-FIBROUS SPONGES. [ May 13, 
with minute spiculated biternate retentive spicula, and a few single 
ones were entangled in the adjoining interstices of the skeleton, As 
the colour of this small portion of the membrane was the same as 
that of minute portions of sarcode dispersed amidst the reticulations 
of the skeleton, there can be no reasonable doubt of its really belong- 
ing to the sponge. 
These spicula are so minute that they require a microscopic power 
of about 700 linear to define them in a satisfactory manner, and in 
the present case they were only visible after having been immersed 
in Canada balsam, A detached specimen of one of these spicula is 
represented by fig. 11, Plate XXII. 
The structure of the skeleton is stronger, larger, and more irre- 
gular than that of I. panicea or I. beatriz; but there is no doubt of 
its being truly an Iphiteon. The average diameter of the skeleton- 
fibre is =4,, inch. The surface-fibres are very closely tuberculated, 
the tubercles looking very like small extraneous patches of silex 
adherent to the surface; and clusters of these coarse tubercles are 
frequently accumulated on the umbones of the confluent areas of the 
skeleton-structures, as represented in fig. 10, Plate XXII., which re- 
presents a portion of the surface of the rigid skeleton. The tubercles 
of the interior fibres are much more regular in their form, and are 
frequently disposed in lines, consisting of five or six of them at 
nearly right angles to the axis of the fibre; and a very considerable 
number of the fibres have no tubercles upon them. 
The rectangulated hexradiate auxiliary fibres were very abundant 
in some of the large interstitial spaces of the skeleton: when fully 
developed they are abundantly spinous, and the radii have spinulate 
terminations ; in an early stage of growth they are frequently spine- 
less, or only incipiently spinous, and in this condition, intermixed 
with the stouter and more developed ones, they may be readily mis- 
taken for spicula; but their habit of anastomosing with each other, 
and their basal connexion with the parent skeleton-fibre, readily dis- 
tinguish them. Fig. 12, Plate XXII., represents two of the auxili- 
ary fibres in a less complicated form than they are usually met with 
in the interstitial spaces of the skeleton, and exhibiting distinctly 
their basement on the skeleton-fibre, and their subsequent inoscula- 
tion. 
The rectangulated hexradiate interstitial spicula are comparatively 
few in number; they are very slender, smooth, and their radii are 
clavate. The auxiliary fibres seem to have superseded them in their 
peculiar office of affording support to the interstitial membranes, 
and of multiplying the sarcodous surfaces of the interstitial spaces. 
The external defensive spicula of the skeleton are remarkably large 
and long. I have not seen an entire one; but in a perfect condition 
they cannot be less than } inch in length, and the diameter of the 
middle of one im situ was =45 inch, more than twice tue size of an 
average-sized skeleton-fibre. Their basal portions are deeply im- 
mersed in the external portion of the skeleton. The basal termina- 
tion in a few cases appeared to be incipiently spinous; but this 
seemed to be rather the exception than the rule. 
