TETHYOI'SILLA. 



Genus TETHYOPSILLA, n. geu. 



Tethyopsillidae of spherical shape ; the whole surface is more 

 or less hairy. Tangential rays of the tetract megasclera very 

 irreyiular and short. No microsclera. 



-^b' 



Tethyopsilla stewartii, n. sp. 



Small, pretty, regularly spherical spouges, which measure only 20 millim. in 

 diameter and are attached by a small base ; broader than high. The speci- 

 mens which I have seen from Mauritius attain a diameter of 40 millim. 

 The surface is even and hairy, in consequence of the presence of projecting 

 spicules. In spirit light flesh-coloured. 



The cortical layer has a thickness of 1*2 millim. Below it tangential canals 

 are met with, which are 0'16 millim. high, and laterally expanded ; they form 

 numerous anastomoses, and form a true subdermal cavity. Narrow inhalants 

 lead down to them. The canals, which extend downward from these subdermal 

 cavities and pervade the pulpa, are, like those of the exhalaut system, narrow, 

 so that the pulpa has a very solid appearance. The exhalants join to form 

 a few oscular tubes 0-7 millim. in diameter, which extend radially and ter- 

 minate in very small oscuJa, which are slit-shaped, V2 millim. long and 0-3 

 millim. wide. These oscula are raised slightly above the surface. 



SheletOH. — (1) Tetract Megasclera. Most irregular. In numbers in the tufts 

 of the cortical layer ; absent in the interior. Angle between the centripetal 

 and tangential rays variable; centripetal ray 2x0-01 millim., straight, conic, 

 pointed ; tangential rays one, two, or three often below the end ; never alike, 

 short, and much curved. It frequently occurs that the angles between the 

 different tangential rays and the centripetal are very different ; one often 

 observes angles of 60°, 120°, and 100° in the same spicule. — (2) Monad Mega- 

 sclera. In the interior abundant, but also forming parts of the surface-tufts. 

 Oxea 1-5 X 0-01 millim., straight, cylindrical, sharp-pointed ; forming bundles 

 and more or less irregular masses, as the centripetal continuations of the 

 surface-tufts. — (3) Tetrad Velar Spicules. Attached to the outer surface by 

 the proximal end of the centi'ipetal ray, which is immersed in the cortex, 

 we find a perfect forest of tetracts ; these project 1-1-5 millim. and have 

 similar dimensions to the cortical tetracts. Their tangential rays are still more 

 variable, however. Two varieties of velar spicules can be distinguished : — 

 (A) Hard Variety : angle between the centripetal and tangential rays generally 

 = 140°, centripetal ray 1-5 x 0-01 millim. ; tangential rays equal, straight or 

 slightly concave outside, sharp-pointed, measuring 0-04 x 0*006 millim ; radial 

 ray always vertical to the surface. These pitchfork-like spicules are fre{[uent 



