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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 95 McAndrewii, with those representing the organization of D. Bowerbankii in Plate VI. figs. 6, 7, 8. The form of the sponge under consideration is that of a shallow cup with expansively undulating margin. The diameter varies from twelve to fourteen inches, and its thickness from half to very nearly one inch. It has six large sinuous doublings of its margin, which extend as much beyond the general plane of the sponge at its under as at its upper surface; two of these foldings of its substance have met at its under surface, and have become cemented together. The greater portion of the sinuous margin of the sponge is flat, the outer and inner edges in some parts being quite sharply defined. The dermal system in this sponge presents very important specific characters. In some sections made at right angles to the surface it was evidently in a state of complete collapse; the under surfaces of the connecting spicula were closely in contact with the surface of the rigid skeleton, and their shafts were deeply immersed in its substance. This position of the expansile dermal system of the sponge is probably its natural one while the animal is in a state of repose. The connecting spicula vary considerably in their size, form, and degree of development. The primary ternate rays are usually short; and the secondary furcating ones are five or six times the length of the primary ones, and without any secondary furcations, while at other times one or more of the furcating rays have a second terminal furcation ; these terminal radii are short, and are frequently projected on a plane at right angles to the other furcations ; these terminal furcations are sometimes very irregular, their apices, instead of two only, having three or four small branches projected in different directions, as represented by fig. 7, Plate VI. The mode of the disposition of the ternate heads of these spicula in the dermis is remarkable: they are not arranged so that their ternate radii form definite inhalant areas ; but the rays cross each other in every imaginable direction, and the pores are found in the little irregular areas, one, or rarely two together, and they therefore appear indiscriminately scattered over the whole of the porous surface. They are simple orifices without any defensive spicula such as we observe in Dactylocalyx McAndrewii. The dermal membrane is abundantly supplied with retentive spicula; they are so numerous and closely packed as to completely obscure it. They are very minute, and no two are alike in size or form; they require a microscopical power of about 700 linear to render them distinct to the the eye. Under these circumstances they present remarkably thick and obtuse proportions, and are distinctly different from any others of this class of spicula that I have ever seen. Sometimes the shaft is multiangulated, each angle producing a single short cylindrical ray, while in other cases the shaft is quite straight, and the radii are projected from it in a perfectly irregular manner. Besides these two prevailing forms, they assume every imaginable variation of shape that such spicula can be subjected to. One of the largest and most regular of the multiangulated forms that I measured presented the following proportions :-length of spiculum l0 l og- inch j |