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Show 1881.J THIi SURVEY OF H.M.S. 'ALERT.' 121 HYMEDESMIA POLITA, sp. n. (Plate X. fig. 9.) Incrusting, 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. Examined. In spirit and by mounting in balsam. In external characters the single specimen is incrusting, very thin (about '6 millim. greatest thickness) ; surface slightly uneven, glabrous, minutely punctate. Colour very dark umber-brown in spirit. Vents chiefly in groups of 2 or 3, oval or circular, opening 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 spicules 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. Length spined very slightly. -2407 mm.; breadth -0038. Length -285 m m . ; breadth •00475. T I Only slightly uneven at base. Spined for at least half of 2. Stout Long I Length-2487mm.; breadth length. Length -25337 Acuate, I .095t mm<. b r e a c j t h .Q0887, 1. Slender Acuate |
