1881.] THE SURVEY OF H. M.S. 'alert.' 123 



drical spicules, which, with loose ones of the same kind, form the 

 dermal skeleton by the crossing of their ends. Main fibre, spicules 

 united somewhat loosely. Parenchyma compact, almost perfectly 

 colourless aud transparent. Skeleton-spicules of three kinds, viz. :— 

 (i) acuate, covered with sparse, short spines for about 4 diameters, 

 from base, tapering to point gradually, size -1647 by '006334 millim., 

 forming basal portion of vertical fibres ; (ii) acuate, smooth, head 

 almost pointed, apex generally somewhat abruptly pointed, size "196 

 by 006334 miUim., forming median portion of vertical fibre ; 

 (iii) cylindrical, smooth, of mainly uniform diameter throughout 

 up to the heads, which are slightly swollen, and then end in more 

 or less sharp hastate points, size -1774 to •18736 by '0038 millim., 

 forming basal and dermal skeleton and summit of vertical fibres. 

 Flesh-spicules, besides the last-named, fine acuates, very slightly 

 blunted at base, tapering to very fine apex; size "152 by "0009 

 millim. ; scattered universally through sarcode. 



Examined. In spirit and by mounting in balsam. 



External Characters. The single specimen, which is extremely 

 well preserved, coats the valves of a Pecten, which was alive when 

 taken. It forms a thin film, varying in thickness from about '70 

 millim. to tissue-paper thickness. It fills up the depressions be- 

 tween the ribs, and thus presents a very smooth rounded contour ; 

 but the lens shows that it is covered with minute points and shallow 

 pits, the former probably representing the terminations of the pri- 

 mary skeleton-columns. Vents are possibly represented by two or 

 three irregular depressions or openings, '25 to '5 millim. in diameter, 

 near the thickest part of the sponge. Pores not found. 



The skeleton is simple in structure, and represents the type as- 

 signed to Hymedesmia by Dr. Bowerbank. The basal lamina is 

 composed of loosely aggregated spicules, about 3 or 4 spicules 

 thick. The composite structure of the vertical fibre is remarkable, 

 and well adapted to secure, by the spination of the basal spicules, 

 solidity of rooting, and, by its shading off into less stout spicules 

 above, pliability. No special cementing sarcode is apparent. The 

 structure of the dermal skeleton is essentially that of the Tedanice, 

 though its connexion with the main skeleton is more marked than is 

 usual in that genus. The parenchyma is slightly yellow, but in 

 the almost entire absence of colour and of opacity resembles that of 

 the Renieridte in general. The two \a,rgeY skeleton-sjncules (acuates) 

 are probably varieties of one original type ; from its position, at the 

 base of the columns with the roughened end downwards, the spined 

 acuate is perhaps developed to suit the incrusting form of the 

 Sponge ; otherwise it differs from the smooth form mainly in being 

 slightly shorter. The cylindricals are really sharply pointed ; but 

 the penultimate swelling is generally discernible, and sometimes gives 

 a fine spear-head outline to the head ; they seem to be a further de- 

 velopment of the typical cylindrical form in the same direction as 

 that shown by Tedania tenuicapitata (sp. n.). 



Ilab. Portland Bay, Chili (opposite the chief island of Madre-de- 

 Dios Archipelago), 10 fathoms. On both valves of small Pecten. 



