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Show 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 Myliusia, St. Vincent's, Rev. L. Guilding, 40. 10. 23. 11." O n the back of the board "Scrivener." 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 sometimes 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. N o sarcodous matter could be detected. There are no visible remains of the expansile dermal system of the sponge. W h e n 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.). W h e n 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 6hort, 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. X X V . ) . |
