REPORT ON THE MONAXONIDA. 13 



square and 18 mm. thick, from the upper surface of which arises abruptly a siugle, 

 stout, thick-walled oscular tube with a large circular osculum at the summit. Height 

 of oscular tube about 25 mm., diameter 14 mm., thickness of wall 4 mm. Colour in 

 spirit greyish-yellow. Texture hard and stony, but rather brittle ; internally rather 

 cavernous. Surface smooth but uneven, and harsh to the touch. Dermal membrane 

 distinct, but firmly adherent to the underlying tissues, translucent, very granular, 

 frequently reduced to a mere network by the enormous number of pores. Oscula, one 

 only present, a little over 4 mm. in diameter, at the summit of a large tubular pro- 

 jection. Pores, very numerous rounded openings about 0'07 mm. in diameter. 



Skeleton. — There is nothing that can be properly termed a distinct dermal skeleton, 

 but the dermal membrane is supported by a reticulation of stout spiculo-fibre, which is 

 not to be distinguished from the main skeleton. The main skeleton consists of a 

 reticulation of very well developed, stout spiculo-fibre in which one distinguishes primary 

 fibres running vertically to the surface and secondary ones crossing them more or less 

 at right angles. This distinction into primary and secondary fibres is not so evident 

 in the deeper parts of the sponge as near the surface, the skeleton arrangement becoming 

 here a more or less confused reticulation of stout spiculo-fibre. The fibre is compact, 

 and averages about 0"1 mm thick. 



Sjncules. — Short, stout, smooth, slightly curved strongyla (PL 11. fig. 14), evenly 

 rounded ofi" at each end, size about 017 by 0'0094 mm. 



This species comes near to Schmidtia aulopora, but is distinguished from that 

 species by the strongylote instead of oxeote spicules, by the stouter skeleton fibre, and 

 possibly also by the arrangement of the oscula. If it should prove that the oscula in 

 our species are always situated on the ends of prominent oscular tubes, this would be a 

 very important character. This Petrosia approaches the most closely of all the Chal- 

 lenger species to that named Reniera crateriformis by Mr. Carter, characterised by a 

 cup- or bowl-shaped form and sausage-shaped spicules. 



Locality. — Station 208, January 17, 1875; lat. 11° 37' N., long. 123° 31' E. ; 

 Philippine Islands; depth, 18 fathoms ; bottom, blue mud. One piece. 



Petrosia variabilis, Ridley, sp., var. (PI. II. fig. 12). 



1884. Schmidtia variabilis, Ridley, Zool. Coll. H.M.S. "Alert," Brit. Mus., p. 41.5, pi. xxxix. 



fig. N, pL xlL fig. t. 



With this species we identify as a variety a single specimen from the Philippine 

 Islands. It is irregularly lobate in form, and, though hard in texture, is very brittle and 

 cavernous ; the surface is uneven and harsh to the touch, and has also a minutely reti- 

 culate appearance. This variety differs from the types of the species in its pale greyish- 



