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Show 334 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [May 13, the rigid skeleton ; but as these tissues on the exhalant surface are not nearly so distinct and regular in their structure as those of the inhalant surface, I could not find a piece that would have afforded a satisfactory figure, although when viewed beneath the microscope the nature and characters of the tissues were beyond a doubt. The furcated foliato-expando-ternate connecting spicula, when thus seen in situ, are so closely packed, and the terminations of their radii are so locked together, that they cannot be separated by the eye ; and the small acerate tension-spicula so profusely scattered on the dermal membrane covering their apices tends greatly to confuse the aspect of the tissues beneath: it is only when we have one of them separated, as represented by fig. 6, Pl. XXIII., that we are enabled to comprehend their structure. But although ineligible for figuring, this fragment of the expansile dermal system clearly demonstrated the agreement in general structure of this species with those in which it is more amply and clearly exhibited. The furcated foliato-expando-ternate connecting spicula are singular in their form, and are very characteristic of the species. Both the primary and secondary ramifications of their apices are very much depressed; they are very thin, and small short branches are projected from their edges so as greatly to increase their plane of support to the dermal membrane, which appears to have closely adhered to them in the living state, as I have not seen any separate spiculum of this form without a portion of the dermal membrane and its numerous tension-spicula closely adhering to its external surface. The rectangulated hexradiate interstitial spicula appear to be few in number in the present condition of the sponge. They are small and slender, and the apices of the radii are sliglitly inclined to be clavate; the axial and rectangulating radii are usually of very nearly the same length,-a few of them only having the basal portions of the axial radii elongated to about twice that of a rectangulated ray. There are two sorts of spinulo-multifurcate hexradiate retentive spicula, with seven or eight spinulate radii to each termination :-one in which the primary radii are short, and the secondary ones projected expansively, so as to form one great compound stellate spiculum, in which it is very difficult to separate with the eye the six sets of terminal spinulate radii; the other form in which the primary radii are longer and the terminal groups of spinulate spicula, usually six, rarely seven or eight in number, are projected contractedly so as to form six separate and very distinct groups of terminal spinulate spicula, as represented by fig. 7, Pl. XXIII. The first-mentioned form is very like that from Dactylocalyx pumiceus, represented by fig. 4, Pl. III., part 1, with the imaginary addition of as many more radii as are there represented. MYLIUSIA, Gray, Proc. Zool. Soc. 1859, p. 439. Skeleton siliceo-fibrous. Fibres solid, cylindrical. Rete symmetrical, disposed in a series of crypt-like layers parallel with the external surface, with intervening planes of perforated siliceous tissue. |
