166 THE VOYAGE OF H.M.S. CHALLENGER. 



Skeleton. — Composed of a very coarse, widely open reticulation of stout, tough spiculo- 

 fibre, brandling and anastomosing, and evidently containing a considerable quantity of 

 horny cementing substance. This fibre is composed of longitudinally placed, very densely 

 packed, slender oxeote spicules, firmly adherent to one another; and is echinated from the 

 surface by spined stylote spicules which project at right angles to the fibre. The fibre 

 itself is commonly about 0"5 mm. in thickness. 



Spicules. — Megasclera; (1) smooth, slender, slightly curved, gradually and sharply 

 pointed oxea, measuring about 0-35 by 0'0063 mm., forming by far the greater portion 

 of the skeleton. (2) Straight, slender, tapering, bluntly pointed, entirely spined styli ; 

 spines small, abundant near the rounded apex, directed towards the base ; size of spicule 

 about 0"17 by 0'0075 mm.; echinating the skeleton fibre. 



In its external form and in its wiry, bristle-like fibre this species difiers very widely 

 indeed from the foregoing, though in spiculation the two come very near to one another. 



Locality. — Papiete Harbour, Tahiti ; depth, 20 fathoms. Seven or eight specimens. 



Family IV. Axinellid^. 



Skeleton typically non-reticulate; consisting of ascending axes of fibres from which 

 arise subsidiary fibres radiating to the surface. Fibres typically plumose. Megasclera 

 chiefly styli to which oxea and (or) strongyla may be added. Microsclera rarely 

 present, never chelae. 



Genus Hymeniacidon, Bowerbank (Pis. XXXIIL, XXXIX., XL., XLV.). 



1864. Hymeniacidon, Bowerbank, Mon. Brit. Spong., vol. i. p. 191. 



Form massive. Skeleton reticulate, composed of ill-defined spiculo-fibre, not plumose. 

 Megasclera all monactinal, styli or subtylostyli. No microsclera. 



Bowerbank's original diagnosis {loc. cit.) runs — "Skeleton without fibre, spicula 

 without order, imbedded in irregularly disposed membranous structure." The genus 

 certainly is a difficult one to diagnose but we must have something more definite than 

 this to go by. Bowerbank's diagnosis includes far too much for any natural genus and 

 its author makes no less than forty-two species of Hymeniacidon out of a total of one 

 hundred and ninety -three species of British sponges.^ 



The original type of the genus is Hymeniacidon caruncula^ and, taking this as our 

 starting point, we have endeavoured to construct a more compact genus with the diagnosis 

 as given above. 



The systematic position of the genus must still be regarded as very doubtful. Having 



*Mon. Brit. Spong., vol. ii. p. xx. ^ yi^ig M.qti. Brit. Spong., vol. i. p. 191. 



