336 DR. J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [May 13, 
fication in loose fasciculi of two to four or five together. Retentive 
spicula spinulo-multifurcate hexradiate stellate. 
Colour of skeleton translucent white. 
Hab. St. Vincent’s, West Indies (Rev. Lansdown Guilding). 
Examined in the skeleton-state. 
The specimen proposed as the type of the genus Myliusia has on 
the front of the board on which it is fixed JZyliusia, St. Vincent’s, 
Rev. L. Guilding, 40. 10. 23. 11.” On the back of the board ‘ Scri- 
vener.”’ 
The sponge is sessile, the base being as wide as the specimen, 
which has a diameter of about three-fourths of an inch, and is about 
half an inch in height. The form of the mass is slightly oval ; it is 
composed of a series of thin sinuous plates of skeleton-structure not 
more than one-third of a line in thickness. The sinuations of the 
plates form deep orifices in the substance of the sponge, which some- 
times extend nearly to the base. By the aid of a lens of an inch 
focus, the stratified texture of the sinuous plates is distinctly visible. 
No sarcodous matter could be detected. 
There are no visible remains of the expansile dermal system of the 
sponge. When viewed by the microscope the surface of the rigid 
skeleton has a very remarkable aspect. It is formed of a series of 
square or irregularly angular areas, the angles of which are filled in 
with thin perforated angle-plates with their inner margins curved, 
so that when combined they leave a large circular or oval orifice in 
the middle of each space; and the upper surface of each layer of 
vaulted structure presents as nearly as possible the same aspect as 
the external layer of the rigid skeleton. There is no uniformity, 
either of size or arrangement, in the perforations of these horizontal 
angle-plates ; but combined they present to the eye the idea of the 
greatest amount of lightness, strength, and beauty that can well be 
conceived to exist in such a structure (fig. 8, Pl. XXIII). 
When we obtain a favourable section of the rigid skeleton at right 
angles to the surface of the sponge, we find that it is formed of a 
series of crypt-like layers of skeleton-fibre, each layer forming as 
it were a distinct and extensive crypt-like space with short, stout, 
cylindrical pillars with gradually expanded bases and capitals, the 
intervening portions of the shafts of the columns being irregularly 
studded with acutely conical incipient spines. Occasionally the 
regularity of the columnar arrangement is broken by the occurrence 
of large irregular interstitial spaces, into which short, stout, very 
spinous cylindrical or attenuating portions of fibre are projected, very 
like the basal portions of the auxiliary fibres that occur in several 
species of Iphiteon, but never appearing to throw off rectangulating 
lateral branches. These organs are evidently rather for defensive 
purposes than as auxiliary supporters of the sarcodous membranes, 
as beside them these spaces frequently have several long and slender 
acerate interstitial spicula traversing them in various directions; 
while in the crypt-like spaces a few only of such spicula are seen 
passing through them in diagonal directions (fig. 1, Pl. XXYV.). 
