450 COLLECTIONS FEOM MELANESIA. 



79. RhaphidopMus arborescens. (Plate XL. fig. L ; 

 Plate XLII. figs, n, n'.) 



Sponge stipitate, much branched, bush-like; branches angular 

 rather than cylindrical ; surface nodular, connected by frequent 

 horizontal trabeculse at right angles to the erect branches. The 

 average diameter of the stem and its branches is 4-5 millim. The 

 cortical incrustation of spicules consists of a layer about -5 millim. 

 thick, the outer part of which consists of loose fascicles of the 

 smooth spinulate spicule, with the pointed ends placed outermost ; 

 the spicules are closely approximated to each other below the 

 surface, between the intermarginal canals, but their distal ends 

 diverge and spread out somewhat at the surface, and between 

 them appear to be placed the pores ; the intermarginal canals, 

 as stated, lie between the bases of these fascicles. The deeper 

 part of this layer consists of Halichondrioid spiculo-fibre, about 6-8 

 spicules broad, with small roundish or polygonal meshes, which seem 

 to have enclosed small canals (probably the afferent canals leading 

 from the intermarginal cavities to . the ciliated chambers). No 

 horny matter is to be seen in this part of the skeleton, the extreme 

 fragility of which forbids the idea that any such occurs here [on 

 the contrary, the compressibility and readiness with which the 

 spicules and fibres must be able to move upon each other in life, 

 owing to the manner of their aggregation, point to a probable great 

 power of contractility and expansion in the dermal membrane, with 

 important consequences to the pores, intermarginal cavities, and 

 inhalent canals which it contains ; and I should anticipate that good 

 spirit-specimens would show the strong development here of muscle- 

 ceUs, such as has been shown by Prof. Sollas in Tetilla (Ann. & 

 Mag. N. H. 1882, ix. p. 155)]. The fibres of the skeleton are ir- 

 regular ; their course is winding, and the distinction between primary 

 and secondary fibres not clear, except at the surface ; here the ends 

 of the primary fibres, which stand out for some distance beyond the 

 general reticulum and support the dermal crust, are absolutely con- 

 cealed by the enormous abundance of points of the spined spicules 

 which project from them. 



Spicules : — (1) Slightly spinulate, smooth acuate, "34 by '0063 ; 

 (2) Spined acuate, slightiy constricted baaally, -08 by -0044; 

 (3)^ Delicate equianchorate, palms proximally square, -012 mUHm. 

 long. 



Hah. Friday Island, Torres Straits. 



The specimen, which is dry, measures 125 mUlim. (5 inches) in 

 height and 60 millim. (2^ inches) in maximum diameter. 



Vosmaer's Olaihria ulmvs (Notes Eoy. Mus. Netherl. ii. p. 151) 

 resembles this species, but is stated to have a bihamate flesh- 

 spicule, and no dermal crust is described. The species differs from 

 B. cratitms, Esper, in the well-branched habit and in minor points in 



