292 PLATE LXXXV. 



and in the size and proportion of its skeleton spicula. 

 The two species have the same open, cavernulous, 

 skeleton structure and the same form of the spicula, 

 but their size and proportions vary to a considerable 

 extent. Those of H. medius being much more slender, 

 and a fully developed one measured but -^y inch in 

 length, while a fully developed one of H. mammeata 

 measured -j^ inch in length, and the diameter was 

 increased proportionally. 



The comparison with N. plumiger was the reverse 

 of that with H. mammeata. The spicula of H. plumiger 

 are purely acuate, and one of the largest measured 

 but yio- inch in length, and the skeleton structure is 

 not nearly so cavernulous, and is very much more 

 delicate in its construction. The two species also 

 differ materially in the characters of their dermal 

 membranes. 



There is no other British species of Hymeniacidon 

 with which the species under consideration is likely to 

 be confounded. 



DESMACIDON INCOGNITOS, Bowerlank. 

 Plate LXXXV. 



Sponge : a fragment only. Surface rough and open. 

 Oscula unknown. Pores inconspicuous. Dermal 

 membrane spiculous ; tension spicula acuate, long, 

 and slender, rather numerous, dispersed, with an 

 admixture occasionally of stout acuate spicula. Skele- 

 ton. Reticulations irregular, rather wide; fibre slender, 

 rather sparingly spiculous ; spicula acuate, variable in 

 size, long, and slender, with an admixture of large 

 stout acuate ones. Interstitial membranes spiculous, 

 the same as those of the dermal membranes, and about 

 as numerous. Gemmules membranous, aspiculous. 



Colour. In the dried state ; brown. 



Habitat. Fowery Harbour ; Mr. C. W. Peach, 



Examined. In the dried state. 



