248 PERCY SLADBN TRUST EXPEDITION 



placed, rounded apex bearing a group of between twenty and thirty minute, sphinctrate 

 vents of varying sizes. The surface is smooth and only slightly uneven ; minutely 

 punctate under a pocket lens owing to the thickly and for the most part uniformly 

 scattered pore-sieves. The texture is rather leathery owing to the well-developed cortex, 

 internally somewhat cavernous owing to the more or less radially arranged canals. Colour, 

 throughout, brown, not very dark (in spirit). A small amount of calcareous debris is 

 attached to the surface but there seems to be little or none internally. The longer 

 diameter of the base measures 34 mm., the shorter diameter 17 mm., and the maximum 

 height from base to apex about 17 mm. 



The cortex is fibrous and about 0"35 mm. thick. It is much excavated, however, 

 by the large inhalant chones, each divided into ectochone and endochone. The ectochone 

 is conical in shape, with the broad base turned outwards and formed by the thin sieve- 

 membrane pierced by numerous inhalant pores. The ectochone is separated from the 

 endochone by a moderately thick diaphragm of fibrous tissue pierced by a single pore 

 which occupies the apex of the cone. The endochones merge insensibly into the large, 

 irregular, subcortical crypts, which unite together in groups and give rise to the wide 

 inhalant canals which penetrate to the interior of the sponge. 



The mesogloea of the choanosome is finely granular, penetrated by numerous narrow, 

 branching canaliculi. The chamber system cannot be satisfactorily made out in my 

 preparations. 



The main skeleton is an irregular feltwork of moderate-sized oxea. These are 

 present abundantly in the fibrous cortex, as well as in the choanosome. Indeed, they 

 almost seem to form a special cortical layer, being arranged paratangentially, while 

 beneath the cortex there is a tendency towards radial arrangement. The radial ones 

 also frequently penetrate the cortex with their outer ends. There are also in the cortex 

 numerous very much smaller, slender oxea, arranged radially and with their outer ends, 

 now mostly broken off, projecting slightly beyond the surface. 



The outermost part of the cortex and the thin, pore- bearing membrane which covers 

 over the inhalant chones are densely charged with minute spherasters ; beneath this layer 

 lie numerous large spherasters, practically confined to the outer half of the cortex, where 

 the fibrous tissue is less strongly developed. Slender-rayed oxyasters, together with 

 a few of the other kinds, occur scattered through the choanosome. 



Spicules: — (1) Oxea (Plate 46, fig. 3a); very slightly curved, fusiform, usually 

 gradually and finely pointed at each end, occasionally blunted or even becoming stylote, 

 and occasionally Avith short abnormal branches. Measuring up to about 0"7 mm. in length. 

 Thickness very variable, up to about 0"025 mm. but usually more slender. 



(2) Slender cortical oxea ; slightly curved ; ends varying from sharply and gradually 

 pointed to rounded off.' Size about 0'2 by 0*004 mm. 



(3) Large spherasters (fig. 3 h) ; with large centrum and conical rays beset with 

 short spines. Total diameter about 0"0287 mm. 



(4) Spherasters (fig. 3 c) ; with small centrum and numerous long, slender rays ; 

 diameter about 0"016 mm. Fairly common in the cortex; possibly young forms of the 

 large spheraster. 



