REPORT ON THE MONAXONIDA. 9 



species for its reception. In spiculation it comes very close to Halichondria panicea, 

 while its general form suggests Rhizochalina. 



Locality. — Station 203, October 31, 1874; lat. 11° 6' K, long. 123° 9' E.; 

 Philippine Islands; depth, 12 to 20 fathoms; bottom, mud. One specimen. 



Genus Petrosia, Vosmaer (Pis. II., III.). 



1862. Reniera (?), Sckmidt, Spong. Adriat. Meer., p. 76. 



1863. Schmidtia, Balsamo-Crivelli, Atti Soc. Ital. Sci. Nat., vol. v. p. 293. 



1864. Thalysias, pars, Duchassaing and Michelotti, Spongiaires de la mer Caraibe, &c. 

 1885. Petrosia, Vosmaer, Bronn's Klass. u. Ordnung. d. Thierreiclis, Porifera, p. 338. 



Sponge usually hard or even stony ; generally with numerous, well-defined, large 

 oscula. Skeleton more or less confused ; spicules oxeote to strongylote, usually short 

 and thick, packed close together in tracts. 



The most obvious feature of this genus is its hard, often stony texture. Vosmaer 

 has changed the older name Schmidtia into Petrosia, because the former was found 

 to be already occupied. 



Petrosia similis, Ridle)^ and Dendy (PI. II. fig. 10 ; PL III. figs. 3, 4). 



1879. Tlialysias suhtriangularis. Carter, Phil. Trans., vol. clxviii. p. 287. 



1886. Petrosia similis, Ridley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, vol xviii. p. 327. 



Sponge (PI. III. figs. 3, 4) variable in external form, repent, branching ; branches 

 sometimes anastomosing, subangular or cylindrical, with large circular oscula arranged 

 in series on the upper surface ; or erect and lobate with large circular oscula at the 

 summits of the lobes. We have here two very distinct types of external form in the 

 same species. The repent specimens (PI. III. fig. 3) are very much broken ; the best 

 measures 93 mm. in length by about 12 mm. in diameter, and bears nine large circular 

 oscula arranged in a single row along its upper surface. There is only one lobate 

 specimen (PI. III. fig. 4), which in external form bears some resemblance to Clathria 

 elegantula, nobis (^cf. PL XXVIII. fig. 3), and measures 81 mm. in height by 87 in 

 width. Colour in spirit yellowish-grey. Texture (of the lobate specimen) firm and 

 almost incompressible, fibrous ; (of the repent specimens) much more yielding to the 

 touch, easUy compressible, fibrous but brittle. Surface smooth but uneven and harsh 

 to the touch ; often with a reticulate appearance owing to the reticulations of the 

 skeleton fibre, which show through the dermal membrane. Denned membrane distinct, 

 translucent, firmly adherent to the underlying tissues. Oscida large, circular, 

 numerous ; sometimes more than 6 mm. in diameter, like great circular pits in the 

 surface. Pores, round oj)enings in the dermal membrane, very few seen. 



(zooL. CHALL. EXP. — PAET Lix. — 1887.) Nnn 2 



