370 Zoological Society : — 



formed of agglutinated particles of silicious sea-sand, this coat 

 being much thicker than the sponge itself; it is probably used to 

 keep it in its place and position at the bottom of the sea. 



Genus Xenospongia. 



Sponge free, discoidal, subcircular, concave below, convex above 

 (rarely lobed on the side) ; the lower surface with a thick coat of 

 agglutinated silicious sand of nearly equal-sized particles ; the upper 

 surface covered with a white leathery coat formed of felted spicula, 

 studded with round tufts of glassy spicula, the tufts of nearly equal 

 size, formed of numerous very fine transparent filiform spicula, form- 

 ing a roundish brush, each tuft surrounded at the base by a slightly 

 raised edge of the leathery upper coat ; the circumference of the 

 disk is surrounded by a uniform series of similar tufts. The centre 

 of the upper surface is marked with a subcentral impressed groove 

 with raised edges enclosing a series of circular oscules ; this groove 

 sends out branches diverging towards the edge, which are forked 

 and reforked (or rarely trifid) as the disk enlarges, until they ap- 

 proach the edge of the circumference, which is surrounded by two 

 continuous circular grooves, concentric with the margin, containing 

 between them a single circular submarginal series of tufts of spicula. 



When the sponge is young, the forked diverging grooves are few, 

 definite, and evenly spread over the surface of the disk, with several 

 series of tufts between them ; but as the sponge increases in age, 

 the grooves become much more numerous, closer together, nearly 

 parallel with each other, and enclosing only a single series of tufts 

 of spicula between the ])arallel grooves. 



The substance of the sj>onge between the grooves is minutely 

 netted, the interspaces of the network being formed of bundles of 

 very minute spicula, with a single series of small uniform-sized, 

 equal, roundish oscules. 



The upper surface of the adult sponge is sometimes taken posses- 

 sion of by a species of Bahmus, which forms a prominence on its 

 surface, and is covered externally with a coat of the sponge. 



Xenospongia patelliformis. 



Hub. Torres Strait. 



The particles of sand forming the lower coat of the sponge are as 

 if they were imbedded in a kind of plaster, having a smooth uniform 

 surface, exactly as if the sand had been well mixed with a small 

 quantity of fluid mortar and then cast upon a smooth body. 



The whole under surface is not perfectly smooth, but with more 

 or less distinct impressed lines or concavities placed parallel with 

 the circumference, showing the periodical increase in the size of the 

 sponge. 



There is scattered over the under surface of the larger specimen a 

 few larger dark-coloured stones and a few fragments of shells, which 

 give a variegated appearance to the coat. The larger specimen, after 



