Oil a HPW Sponge from Chn'sfmas Island. 177 



XVI 11. — Oil a new Lithonine Sponge from Christmas Island. 

 By K. KlRKPATRICK. 



\VilEN" looking ovlt .<o!iie pieces oF rock dredged by me from 

 50 fiitlioms otf ('liristmas Island, I came acro.ss two thin, 

 vitreous-wliite ciiiots, whicii were found to be Lithonine 

 sponges. 



Tlie larger crust, wliicli covers an area of about 35 milli- 

 metres, is about 0'7o mm, thick in the centre, fining away 

 to a verv thin edge. When magnified, the surface shows a 

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

 conical spikes rising up veiticaliy about 0*12 mm. At the 

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

 <lense 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 the 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, 



Tlie loose spicules are mostly monaxons, curved at one 

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

 at the otiier. The average size is 120 x 3*5 /j,. 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 

 Plectroninia, 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 Murrayona (Proc. 

 Roy, kSoc, 1910, p, 124) I divided the family Piiaretronidge 

 into three subfamilies, Dialytinge, Lithoninre, and Murrayo- 

 nin^j and stated that my classification was the same as tiiat 



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



