1881. j THE SURVKY OF H. M.S. 'ALERT.' 121 



Hymedesmia polita, sp. n. (Plate X. fig. 9.) 



lucrusting, thin. Surface glabrous, with minute scattered points, 

 dark umber-brown. Vents chiefly grouped two or three together, 

 minute. Pores scattered. Main skeleton of short primary spicular 

 columns extending directly from base to surface, which break into a 

 slight brush just below surface, and slightly project from it ; bases 

 surrounded by groups of small spined acuate spicules. Dermal 

 skeleton of a thin loose spiculo-fibre connecting the primary columns. 

 Sarcode reddish brown. Main skeleton-spicules of two kinds : — 

 (i) Spined acuate, spines reaching to within one fourth of length of 

 the sharp apex, most strongly developed at base, size •25337 by 

 •00887 millim. ; (ii) smooth acuates tapering from head almost to 

 the apex, which is abruptly pointed, size -2407 by -0038 millim. 

 Dermal skeleton-spicules same as latter. The small spined acuates 

 are entirely spined, size • 10135 by '006334 millim. Flesh-spicules 

 confined to dermis, of one kind, viz. equianchorates in rosette-like 

 groups, shaft slender, front palms entire, with a straight lower 

 edge, tubercle prominent, length •01267 millim. 



Exatnined. In spirit and by mounting in balsam. 



In external characters the single specimen is incrustiug, very 

 thin (about •G millim. greatest thickness) ; surface slightly un- 

 even, glabrous, minutely punctate. Colour very dark umber-brown 

 in spirit. Vents chiefly in groups of 2 or 3, oval or circular, open- 

 ing obliquely to surface ; diameter about ^25 millim. Pores oval, 

 scattered, numerous, about '07 millim. in greatest diameter. 



Skeleton. No distinct basal membrane. Some lines of fine 

 long acuates lie at the base. A set of distinct primary spicular 

 bundles spring from the base at from "18 to "36 millim. apart; their 

 bases are surrounded by groups of spined acuate spicules ; they 

 proceed to surface approximately at right angles to it; and their spi- 

 cules diverge laterally, echinating the fibre until just below the 

 surface, where they diverge slightly ; the apices of the terminal 

 spicules project beyond the surface slightly. 



Dermis. Lines of fine long subparallel acuate spicules extend 

 between the apices of the primary skeleton-columns, diverging from 

 one another where the lines are bent. 



Hab. On a Balanus sessile on large worm-tube. Sandy Point, 

 7-10 fathoms. 



Obs. It approaches Microciona tuberosa, Bowerbank, from the 

 Straits of Malacca, very closely in spiculation and some other 

 characters. 



Microciona tuberosa, 'Bowk. Hymedesmia polita. (Ma- 



(Straits of Malacca.) gellan.) 



Sometimes basally micro- Always smooth. Lengtli 



spined very slightly. •2407 mm.; breadth '0038. 

 Length -285 mm. ; breadth 

 ■00475. 



Q, , , J- ( Only slightly imeveu at base. Spined for at least half of 



J,,, ^°^^^ \ Length •2487mm.; breadth length. Length -25337 



Acmte. I .pg_._ ,^jjj_ . breadth -00887. 



L Slender Actiate 

 Spimde. 



