On a new Sponge from Christmas Island. .177 



XVIII. — On a new Lithonine Sponge from Christmas Island. 

 By R. KiRKPATRICK. 



When looking over some pieces of rock dredged by me from 

 50 fathoms off Christmas Island, I came across two thin, 

 vitreous-white crusts, wliicli were found to be Lithonine 

 sponges. 



The larger crust, wliich covers an area of about 35 milli- 

 metres, is about 0*75 mm, thick in the centre, fining away 

 to a very thin edge. When magnified, tlie surface shows a 

 network of circular holes 0*09 mm. in diameter, and sharp 

 conical spikes rising up vertically about 0"12 mm. At the 

 bottom of the holes or pits and at the edges of the sponge a 

 dense white crust is visible. No surface membrane or soft 

 tissues remain, but loose spicules are visible, imbedded in 

 the skeleton. 



The latter is constructed of fused four-rayed spicules, the 

 apical rays of which form the long conical surface spines, 

 and the other three facial rays the walls of the pits. The 

 dense basal crust is formed of much smaller, four-rayed, 

 densely packed spicules. The facial rays are cylindrical 

 and end each in a circular articulating surface. The under 

 surface of tlie crust has almost the appearance of a mosaic, 

 the facets being formed by the articular ends of the facial 

 rays of the small four-rayed spicules. 



The loose spicules are mostly monaxons, curved at one 

 extremity, running to a sharp point at one end and rounded 

 at the other. The average size is 120 x 3'5 /a. One spicule 

 appears to be bifurcated at one end, and may be regarded as 

 a three-rayed form. 



The sponge seems to me to belong to a new species of 

 Plcctroninia, Hinde, and I propose to call it Plectroninia 

 deansii, after Captain J. Deans, by means of whose skill the 

 specimens were obtained. 



Three other species of this genus are known, viz. : 

 P. halli, Hindcj a knob-shaped species from the Eocene of 

 Victoria ; P. hindei, Kirkp., a thin incrusting form from 

 50 fathoms off Funafuti ; and P. assindice, Welter, a knob-like 

 species from the Greensand of Essen. 



The distinguishing feature of P. deansii lies in the 

 chaiacter of the loose spicules. 



In a paper on the Pharetron Sponge Murraijona (Proc. 

 Roy. Soc. 1910, p. 124) I divided the family Piiaretronidge 

 into three subfamilies, Dialytinge, Lithonin?e, and Murrayo- 

 ninse, and stated that my classification was the same as that 



Ann. & Mag. N. Hist. Ser. 8. Vol. viii. 12 



