38 STELLETTA. — PSAMMASTEA. 



Stelletta tethyoides, n. &p. 



JSpherical sponges, which are attached by a broad base and attain a diameter 

 of 50 millim., with a depression on the upper side about 5 millim. deep, at 

 the bottom of which large oval oscula measuring G X 3 millim. are situated. 

 In smaller specimens the vents are smaller. Colour in spirit dark brown 

 on the outer surface, light brown in the interior. The spherical shape is 

 rendered somewhat irregular by numerous slight irregularly scattei'ed protu- 

 berances. The surface is shagreen-like. The chonte of the inhalant sj^stem 

 approach the shape of ordinary subdermal cavities ; they appear as tangential 

 canals extending below the surface and measure 0-25 millim. in diameter ; 

 large radial canals, which are for 3 to 20 millim. straight and 0*5 to 1 millim, 

 wide, extend in a centripetal direction downwards from these. The exhalant 

 canals join to form large oscular tubes, with a diameter of 2 millim. For 

 about 20 millim. below the surface of the oscular area the space is taken up 

 by a lacunose tissue. 



Skeleton. — (1) Tetract SaiyportiiKj-Siyicides. These are found in the subcor- 

 tical layer only. Angle between the centripetal and tangential rays = 100''; 

 centripetal ray 1-2 x 0-016 millim., straight, smooth, pointed ; tangential rays 

 0-16 X 0-012 millim., slightly curved, conic, not sharply pointed. — (2) Monad 

 Spimles of the Pulpa. Oxea, slightly curved, sharp-pointed, 1-2 x 0-012 millim., 

 numerous, near the outer surface in bundles, towards the interior more irregu- 

 larly disposed. A few styli of similar dimensions also occur in the interioro 

 — (3) Stellate Spicules of the Cortex. A cortex 0*15 millim. in thickness covers 

 the whole of the sponge, with the exception of the oscular area. This cortex 

 is pervaded by the narrow and slender ectochona), and composed of a dense 

 mass of stellate spicules, which have 10 to 15 conic rays. These spicules 

 have a diameter of 0-05 millim. ; each ray is about twice as long as thick at the 

 base, measuring 0-016 millim. in length. — (4) Stellate Spicules of the Pulpu. 

 There are three kinds of stellates in the interior of this sponge : — (A) With 

 few cylindrical, exceedingly slender rays, measuring 0-032 x 0-002 millim. ; the 

 central mass of this spicule is exceedingly small. (B) Small, stout, stellate 

 spicules, with numerous sharp and conic rays, measuring 0-016 millim. in 

 diameter. (C) Large, stout stellates, with numerous conic rays, measui-ing 

 U-U64 millim. in diameter. 



Geographical Distribution. — Solomon Islands {Australian Museum, 

 Sydney), 



Genus PSAMM ASTRA. 

 Stellettidye with stellate microsclera and spiiied rods. 



