430 COLLECTIONS FROM MELANESIA. 



others the secondaries of the main skeleton. Sarcode very pale 

 brownish yellow, thin and transparent. 



Spicules : — (1) Skeleton acuate, smooth, straight or slightly curved, 

 with rounded base slightly smaller than the middle of the shaft, 

 which tapers gradually to a fine point from near the middle of the 

 spicule: size of spicule "18 to *2 by '046 to *0063 millim. : in 

 middle of fibre. (2) Flesh-spicule, equianchorate, navicular, shaft 

 gradually curved, slender ; length of spicule '013 to - 016 millim. 



Hub. Thursday Island, Torres Straits. 3-6 fms. (on bivalve shell 

 and Hydroid). 



Represented by a dry specimen and by one in spirit ; the larger 

 one is 50 millim. high by 55 in extreme diameter. The species 

 is distinguished by its regularly rectangular main skeleton and 

 well-developed horny fibre. 



65. Myxilla arborescens. 

 (Plate XL. fig. G; Plate XLII. figs, a-a".) 



P Halichondria plumosa, Carter, Phil. Trans, vol. 1G8, p. 287 (nee 

 Spongia plumosa, Montagu, Wern. Mem. ii. p. 116). 



Erect, pedicellate, branched, branching not confined to one plane, 

 forming "heads" by the aggregation and partial anastomosis of 

 many different pedicellate branched growths arising from one or 

 more common stems ; mode of branching dicho- to pollaeitornous *. 

 Stems, both primary and secondary, slender, of angulated outline, 

 owing to the lateral projection from them of a number of prominent, 

 jagged, longitudinal ridges. Branches palmate, the edges sharp, the 

 flat surfaces covered with longitudinal, very prominent ridges and 

 upwardly projecting points, the tips of the branches subtruncate. 

 Thickness of secondary stems, exclusive of surface-projections, about 

 1*5 millim., of palmate parts of branches -2o to - 5 millim. Minute 

 appearance of surface in spirit granulated (i. e. covered with minute 

 rounded elevations, which are smooth and glabrous in spirit). 

 Texture in spirit tough, very pliable, of very imperfect elasticity. 

 Vents apparently represented by round or oblong apertures, -25 

 to 1 millim. in maximum diameter, numerous, placed between 

 prominences of surface of branches. Colour in spirit dull pale 

 brown. Sarcode rather granular, pale yellow-brown, soft. Skeleton 

 consisting of longitudinal lines of loosely aggregated spicules (nos. 

 1 and 2), about 8 to 10 spicules broad, surrounded by some loose 

 spicules of the same kind and echinated by spicule no. 1 ; the 

 lines run approximately parallel with each other, occasionally 

 branching and anastomosing at acute angles : at the surface these 

 primary lines either become loose and form loose tracts of skeleton- 

 spiculcs running along the surface, or they remain compact and 

 project as surface-tufts. 



Skeleton-«picules : — (1) Spined acuate, tapering gradually from 

 rounded head to sharp point, generally somewhat curved ; spines 



* i. e. dividing many times at one point; from iroWdicis. 



