150 Prof. W. J. Sollas on the 



1872, Tethyn cranium, Carter, Ann. & Mag. Nat. Hist. vol. ix. p. 419, 



pi. xxii. fig. 9. 

 1874. Tethya cranium, Bwk. Monog. Br. Sp. iii. p. 315, pis. xiv. & 



Ixxxix. 



This interesting sponge, the occurrence of which in the 

 Norwegian seas had been early noticed by Bishop Pon- 

 toppidan (Lanix. loc. cit.), is well represented in Mr. Nor- 

 man's collection by several small but perfectly preserved 

 specimens. A clear insight into its exquisite structure is 

 afforded by a series of thin slices obtained by means of the 

 freezing microtome, and mounted in balsam or glycerine. My 

 chief regret is that its beauties have not found a more skilful 

 pencil to portray them. 



The sponge is approximately spherical in form, white, and 

 with a hairy appearance due to the projection of the ends of 

 the spicular fibres beyond its general surface. An oscule is 

 clearly present, though Bowerbank and other observers have 

 failed to find it. On this point Bowerbank is most explicit ; 

 his specimens were some two hundred in number, and, after 

 careful searching, he could discover no trace of an oscule, 

 pores, or intermarginal cavities, all of which in Mr. Nor- 

 man's specimens can be easily demonstrated. Carter, it 

 should be added, has called attention to the presence of a 

 group of oscules in a specimen which came under his observa- 

 tion (Ann. & Mag. Nat. Hist. ser. 4, 1872, vol. ix. p. 419). 



The oscule is a nearly circular opening, usually small (from 

 0*03 to 0*07 inch diameter), obliquely terminating a tunnel- 

 like tube (fig. 6, 0, transv. sec.) which runs for a short distance 

 along the surface of the sponge ; the tunnel-like roof of the 

 tube is a thin imperforate membrane formed by the extension 

 of the dermis ; the floor is the ordinary dermis of the general 

 surface, which retains its pores, and by their over-develop- 

 ment assumes the character of a fenestrated membrane or 

 network with round meshes. 



The skin or dermal membrane rises tent-like about the ends 

 of the projecting spicular fibres, and extends continuously 

 from one to another, so as to completely invest the sponge. 

 It is best seen by cutting off the ends of the projecting 

 spicules, and viewing under an inch lens by reflected light 

 (tigs. 15, 16). One can then observe shining through the 

 surface of the skin a number of thread-like fibres, which 

 radiate outwards and downwards from the circumference of 

 each spicular fibre as a centre, branching as they go, and 

 anastomosing with those of adjacent centres to form a poly- 

 gonal network, by which the overlying dermal membrane is 

 mapped out, as it were, into a number of polygonal areas. 



