70 CEYLON PEARL OYSTER REPORT. 



they are quite smooth and measure about - 35 millim. by - 018 millim., but with 

 considerable variation in size. Between the large and small oxea thus described we 

 find so many intermediate in shape and size (f-q) that it is impossible to distinguish 

 them as megascleres and microscleres respectively. 



(3.) Oxyasters (Plate II., fig. 3, v -x) with smooth, slender, sharp-pointed rays and 

 little or no centrum, are very abundant in the deeper parts of the sponge ; they 

 measure about 0'02 millim. in total diameter. 



The irregular arrangement of the short-shafted trisenes (which presumably 

 originated phylogenetically at the surface of the sponge and acquired their 

 characteristic form in direct relation to that position), and the absence or scarcity of 

 typical calthrops, seem to indicate that this species is less primitive than the type of 

 the genus [P. copiosa, Schulze), and, in fact, somewhat degenerate. 



B.N. 149 (from Station XLL, 12 miles off Galle, 100 tathoms). 



Stceba, Sollas. 



Thin, encrusting Pachastrellida?, with short-shafted triaenes, resembling calthrops, 

 for megascleres, and only spined microxea for microscleres. 



In 1888 Sollas (loc. cit., p. 102) proposed the genus Stceba for Carter's Samus 

 simplex from the Gulf of Manaar, but Lendenfeld (13), followed by Topsent (14), has 

 merged the genus in Dei cit us. Inasmuch, however, as the type species of Detritus 

 (D. bucklandi, Bk. sp.) possesses toxa amongst its microscleres, while Stceba does not, 

 it seems to me desirable to retain the distinction between the two, especially as we 

 now know thi'ee more or less distinct species without toxa, viz., Stceba simplex 

 (= Samus simplex, Carter; Dercitus simplex, Topsent) ; Stceba plicata (= Coriicium 

 plicatum, Schmidt ; Calcabrina plicata, Sollas ; Dercitus plicatus, Lendenfeld and 

 Topsent), and Stceba extensa, n. sp. 



Sollas's genus Calcabrina, founded for Schmidt's Corticium plicatum, must be 

 regarded merely as a synonym of Stceba. 



Stceba extensa, n. sp. Plate V., fig. 1. 



Sponge thin, encrusting, spreading extensively over the surface of and into the 

 cavities of a mass of calcareous debris (Melobesia, Polyzoa, Coral, Worm-tubes, &c, 

 mixed together). Surface for the most part smooth and sub-glabrous, slightly rugose 

 in parts ; consistence tough and rather fleshy ; colour in spirit, pale grey. Vents and 

 pores not seen. The exposed part of the sponge forms an almost uninterrupted sheet 

 about 45 millims. by 33 millims. in extent, but of irregular outline. 



The main skeleton consists of short-shafted trisenes scattered abundantly, but quite 

 irregularly through the choanosome, but very sparsely in the ectosome, and there is a 

 thin dermal crust of microxea. 



Spicules. (1.) Triaenes (Plate V., fig. 1); with short, stout, sharp-pointed shaft, 

 measuring about - 136 millim. by - 02 millim. when fully developed, with cladome 



