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Show 1876.J SILICEO-FIBROUS S P O N G E S . 539 spines, and they are then generally based on the skeleton-fibres at some point between two of the angles of the skeleton-rete. In other cases four long slender spines are projected in opposite directions from the skeleton-fibres, so disposed as to form a symmetrical cross. In fact, there appears to be no end of variations in the form and mode of disposition of these defensive organs; and they are at the same time exceedingly numerous and very irregularly distributed. At some parts they are disposed singly at intervals, while in other parts a considerable number are congregated within a small space. Although the discriminative characters in the specimen of this sponge are in this instance so few in number, they are fortunately so striking, and so different from those of other nearly allied species, that there will be little difficulty hereafter in recognizing the species in a more perfect condition by the peculiar specific characters afforded by the skeleton and its elaborate system of armature. FARREA IRREGULARIS, Bowerbank. (Plate LVII. figs. 3, 4.) Sponge laminar, cup-shaped ? Surface even. Oscula and pores unknown. Dermal membrane thin, aspiculous. Skeleton siliceo-fibrous ; fibres cylindrical, irregular in size; rete occasionally rectangulated, but more frequently irregular ; central canals very slender, often obsolete. Interstitial defences rectangulated sexradiate, few in number, very small. Sarcode dark, opaque, aspiculous. Colour, in the dried state, dark amber. Hab. Algiers (Mr. Henry Deane). Examined in the dried state. The only specimen that I have seen of this species is a thin plate of it, eight lines in length by five in breadth, which, from its slight curvature in one direction, has apparently formed part of a cup-shaped sponge. I am indebted to m y friend Mr. Charles Tyler for it; and he informed me that it was presented to him by our late friend Mr. Henry Deane, and that its habitat was Algiers. The structures of the dermal surface are even, but much complicated ; and the intervening skeleton-tissues to a great extent have the same character. A few very minute portions of the dermal and interstitial membranes remained, upon each of which there is a thin film of dark amber-coloured sarcode ; but I could not, with a power of 100 linear, discover the slightest remains of retentive or defensive spicula upon any of them. Small irregular dense masses of opaque sarcode are adherent to some portions of the interstitial skeleton ; but they also appeared to be quite destitute of retentive spicula. The configurations of the skeleton of both the inhalant and exha-lant surfaces are very irregular in the mode of the disposition of their skeleton-structures ; and the fibres also of which they are composed are very unequal in size. For small spaces the rete sometimes assumes a quadrangular form ; but the more general mode is very irregular. The skeleton consists of several layers of reticulate structure ; but |