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Show 1875.] DR. J. S. BOWERBANK ON THE SPONGIADcE. 285 The tricurvate acerate tension-spicula are also very small and slender one of the largest of them measured jfa inch in length : the curves of these spicula are not very strongly produced, and some of them exhibit but faint traces of curvature ; they are not very numerous. The retentive spicula are especially remarkable for the great variety of their size and form. The largest of the retentive spicula, the inequidentato-palmate anchorate ones, are congregated in rosette-shaped groups, a few only appearing singly located. The groups of these spicula so characteristic of the genus Baphiodesma are not very numerous, nor do they contain so many spicula as those we observe in some other species of the genus; the usual number is about eight or ten in each. They are situated on the basal membrane as well as on the inner surface of the dermal one, just as similar groups of such spicula are dispersed on the inner surface of our British species R. lingua, in which they are frequently crowded together in considerable numbers. The spicula in the species in course of description are strongly produced; an average-sized one measured -^jtj inch iu length, and the breadth of the broad spatulate distal end was yriVo inch. These dimensions contrast strongly with those of the numerous small inequipalmate anchorate ones dispersed on the surface of the membranes, the average length of which is y-rVg-inch ; and the contrast is still greater with the very minute palmato-equianchorate ones, two of which measured respectively -g-^Vg- and 3"TMJTT in(?b hi length ; so that a considerable number of the latter ones might be shovelled up as in a scoop by one of the large form of these spicula which compose the large rosette-like groups. One of the stout bidentate equianchorate spicula measured -^^ inch in length. A similar difference in size occurs between the two sorts of bihamate spicula. The larger of the two measured 3-^ inch in length, with greatest diameter of the shaft 3^4 inch, while the length of one of the smaller description of this form was j-gVo inch. The fasciculation of the skeleton is very variable ; some of the bundles contain numerous spicula, while others consist of two or three only ; and the mode of disposition of the fasciculi is also very irregular. The spicula in all the bundles appear to have their bases and apices coincident. The length of an average-sized skeleton-spiculum is -£$ inch, and the diameter -g-^Vr inch. The basal membrane is abundantly armed with numerous attenuato-acuate entirely spined internal defensive spicula projected at right angles to its inner surface. In the basal portion of the sponge a thin stem of a Tubularia was imbedded; and this was closely surrounded by basal membrane, from all parts of which numerous internal defensive spicula were projected. This curious habit of the sponge to surround included extraneous matters with membrane, and then to project defensive spicula from their surfaces, I have several times observed in some of our British sponges. On the surface of another sponge from the same locality I found a very young specimen of the sponge under description not exceeding a | of an inch in diameter and as thin as a piece of paper. The dermal membrane was in a fully developed condition; the basal |