187(3. J SILTCEO-FIBROUS SPONGES. o39 



spines, and they are then generally hased 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 rect- 

 angulated, but more frequently irregular ; central canals very slender, 

 often obsolete. Intei'stitial defences rectangulated sexradiate, tew 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 my 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 compli- 

 cated ; and the intervening skeleton-tissues to a great extent have the 

 same character. 



A few very minute portions of the dermal and interstitial mem- 

 branes 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 



