236 PERCY SLADEN TRUST EXPEDITION 



The skeleton is dense, radially arranged, and the spiculation very typical, as 

 follows : — 



(1) Orthotrisenes ; with simple unbranched cladi ; shaft straight, tapering gradually 

 to a sharp point, measuring about 076 by 0"026 mm. ; cladi gradually sharp-pointed, 

 measuring about 0"15 by 0"026 mm. 



(2) Anatrisenes ; numerous and frequently projecting beyond the surface. Cladi 

 strongly recui^ved. Shaft very long and slender, measuring about 0'9 by 0"0086 mm. 

 Cladi gradually sharp-pointed, about 0"034 mm. long. 



(3) Oxea ; straight or nearly so, fairly gradually and sharply pointed, measuring 

 up to about 0"9 by 0'02 mm. Considerably shorter and more slender forms also occur. 



(4) Chiasters (tylasters) ; very minute, with very slender rays and very small 

 heads; total diameter about O'OOS mm. Scarce. 



The cortex is very feebly developed and not sharply differentiated from the choano- 

 some ; say about 0'12 mm. thick. It contains very little fibrous tissue and the large 

 subcortical crypts push their way through it to within a short distance of the surface, 

 lying between the distal portions of the bundles of large orthotrisenes, whose cladi are 

 extended actually at the surface. The inhalant pores seem to open singly by short, 

 narrow canals into the subcoi'tical crypts. An inner zone of smaller orthotriaenes extend 

 their cladi beneath the subcortical crypts. 



The " Sealark " fragment agrees closely with the type of the species from the Red 

 Sea, as described by Row. I have examined one oF Mr Row's preparations of the type 

 and can find no impoi^tant difference. I cannot find the slender, hair-like oxea which 

 he describes and figures, but which I cannot regard as of any taxonomic importance. 



The species is evidently closely related to SoUas's Myriastra simplicifurca [1888] 

 from Torres Strait ; differing, however, in the much smaller size of the spicules. It 

 also comes near to Hentschel's Stelletta tuherosa [1909] from S.W. Australia, from which 

 it differs in the form of the cladome of the anatrieene, the cladi being, usually at any rate, 

 much more strongly recurved. Hentschel also mentions the occurrence of small, slender 

 oxea here and there in the choanosome in his species. They are probably merely young 

 individuals of the large oxea. Probably all three forms will have to be united as varieties 

 of one and the same species, but it would be premature to do this at present. 



Previously knoivn Distribution. Red Sea (Row). 



Register No., Locality, c&c. lv. 1, Coetivy. 



- 5. Myriastra cavernosa n. sp. 



(Plate 44, figs. 3, 3a; Plate 46, fig. 1.) 



Sponge (Plate 44, figs. 3, 3 a) massive, irregularly subspherical ; without definite 

 points of attachment but inore or less thickly encrusted with nuUipores and Orbitolites. 

 Surface uneven, granular, occasionally hispid where well protected. A few rounded 

 openings, say about 3 mm. in diameter, irregularly scattered between the debris on 

 the surface, and without prominent margins, probably represent the vents. They com- 

 municate with the extensive system of wide canals which ramify all through the interior 



