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



towards the outer end, before whicli, however, they become again enlarged and end in a 

 bideutate crescent-shaped anchor. The two teeth of the latter rise from the gently bow- 

 shaped, rounded and thickened terminal portion (PI. XLV. fig. 9), and end in simple 

 conical points. The distance of these two terminal points, i.e., the total breadth of the 

 anchor, is about 0*5 mm. 



Genus 3. PoUopogon, Wyville Thomson (Pis. XLVII.-L.). 



Literature and History. — In his preliminary account of part of the Challenger 

 Expedition ^ Wyville Thomson described a large sponge — PoUopogon amadou — which 

 was found to the south of the Canary Islands at a depth of 1525 fathoms. This 

 he regarded as type of a new genus — PoUopogon. This sponge forms an oblique and 

 upward directed, semi-involute plate, with sharp upper and lateral margins, having the 

 general shape of a tree fungus. From the transversely truncated base a strong beard 

 of long siliceous fibres projects, and these fibres bear on their extremities two 

 widely extended anchor teeth, which serve for fixing the sponge. An uniserial 

 fringe of fine, straight, parallel, projecting siliceous spicules adorns the sharp free 

 side and upper margins. The concave inner and the convex outer surfaces are 

 covered by a fine network with quadrate meshes. " The sponge when brought up was of 

 a delicate cream colour ; it was necessary, to steep it in fresh water to free it from salt, 

 and the colour changed to a leaden grey." 



Character of the Germs. — The body has the form either of a thick-waUcd goblet, or of 

 an ear-shaped involute plate. It exhibits a broad basal tuft, and an oscular fringe of 

 marginalia, but no laterally projecting pleuralia. The parenchyma contains small, 

 extremely rough or spinose, oxyhexacts and uncinates, and in one species even small 

 smooth oxydiacts of varying size and in varying abundance. The two teeth of the basal 

 anchors are disposed approximately at right angles to the long, almost smooth shaft. 

 The marginalia end externally in club-shaped thickenings. 



1. PoUopogon amadou, n. sp. (Pis. XLIX., L.). 



South-west from the Canary Islands (Station 3, lat. 25° 24' N., long. 20° 14' W.), from 

 a depth of 1525 fathoms and hard ground, a beautiful PoUopogo7i form was dredged. 

 This type, which was in 1877 figured and shortly described under the above title in 

 Wyville Thomson's Atlantic, has not the cup shape characteristic of most other 

 Hyalonematidse, but exhibits rather the form of an ear, or that of a leaf rolled up into a 

 semi-funnel (PI. XLIX.). There is thus no gastral cavity but only a^concave gastral, and 

 a convex external surface, both of which are quite smooth, without radially projecting 



1 The Atlantic, vol i. p. 174-176. 



