BRITISH SPONGIAD^E. 321 



in height, and is apparently two specimens, which have 

 united by approximation, as they adhere only by a thin 

 perpendicular plate of tissue. The larger portion is mas- 

 sive, and has several large terminal oscula on its crest. 

 The smaller portion consists of two large fistulae cemented 

 together, the mouth of the largest being nearly five lines in 

 diameter. The surface has a somewhat coarse and open 

 texture, in consequence of the abundance and, compara- 

 tively speaking, large size of the intermarginal cavities ; but 

 the pores in the dermal membrane above these cavities are 

 not visible without the aid of a microscopical power of 

 about 100 linear. 



The tension spicula of the dermal and interstitial mem- 

 branes are about as long as those of the skeleton, but they 

 are not more than half or one third of their diameter. 

 The external defensive spicula are continuations of the 

 primary lines of the skeleton, the spicula of which, when 

 projected beyond the dermal membrane, diverge slightly 

 from each other, forming groups of external defensive 

 spicula. 



The retentive spicula of this species are of a very singular 

 form. They have the curve of the shaft of the spiculum 

 abruptly reversed at its middle, for about one third of its 

 length, so that the central portion of the bow becomes 

 slightly incurvate instead of excurvate ; and at each of the 

 two points where this change of the line of its direction is 

 effected, there is a slight spur projected. These spicula 

 are very minute, and they require a power of about 1000 

 linear to define their structure in a satisfactory manner. 



Mr. Norman's specimens were dredged off St. Martin's 

 Point, Guernsey, on rocks, in about 30 fathoms water. 



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