12 THE VOYAGE OF H.M.S. CHALLENGER. 



Petrosia similis, var. compacta (PI. II. fig. 13 ; PI. III. fig. 5). 



Sponge (PI. III. fig. 5) cylindrical, ramose, erect (?), or repent (?). TTie single 

 specimen in the collection consists of a stout, irregulai'ly cylindrical basal (?) portion, 

 which bifurcates at one extremity into two but slightly divergent, more slender branches, 

 about equal in length to the original piece. Total length 143 mm. Diameter of 

 unbranched portion about 18 mm., and of branches about 10 mm. Colour in spirit 

 light brownish-yellow. Texture stony hard but brittle. Surface uneven, smooth in 

 appearance, but harsh to the touch. Dermal membrane rather coarse, translucent, 

 allowing the subjacent skeleton reticulation to show through. Oscula large, conspicuous, 

 circular, with their margins level (or almost so) with the general surface of the sponge, 

 diameter about 4 mm. Pores scattered through the dermal membrane. 



Skeleton. — The main skeleton is a very dense and compact, but rather irregular 

 reticulation of spiculo-fibre and spicules, in which primary fibres are readily distinguishable, 

 running vertically to the surface. Seen from the surface, the uppermost layer appears 

 as a reticulation of stout spiculo-fibre with rounded meshes about 0-35 mm. in diameter, 

 on which the dermal membrane rests directly. 



Spicules. — Short, rather stout, slightly curved, fairly gradually and fairly sharply 

 pointed oxea (PI II. fig. 13), measuring about 0-22 by 0-0145 mm. 



This sponge is not unlike Schmidtia variabilis, var., from the same station, in 

 external form, but may at once be distinguished by the much smaller spicules and denser 

 texture. It is distinguished from the type of the species by its much more compact 

 structure, and by the absence of the tufts of spicules supporting the dermal membrane, 

 but agrees weU in the size of the spicules. In its stony texture it resembles Petrosia 

 dura, Nardo, but differs from typical specimens of that species in its external form, 

 which is ramose, instead of tuberous or tuberously ramose. The skeleton-fibre also is 

 by no means so compact and well developed, and the meshes of the main skeleton are 

 much smaller. The spicules, again, are smaller, and do not exhibit that remarkable 

 variability in size which characterises those of Petrosia dm-a.^ 



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

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



Petrosia truncata, Ridley and Dendy (PI. II. fig. 14 ; PI. III. fig. 1). 



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



Sponge (PI. III. fig. 1) massive, sessile. The single specimen in the collection is 



unfortunately only a fragment. It consists of a massive basal portion about 25 mm. 



1 Our knowledge of Petrosia dura, enabling us to make the above comparisons, is derived chiefly from specimens 

 in the British Museum, obtained from Professor Schmidt. For descriptions of the species, vide Nardo, his, 1833, 

 p. 522 ; Schmidt, Spong. Adriat. Meeres, p. 76, Taf. vii. fig. 13 ; Balsamo-CriveUi, Atti Soc. Ital. Sci. Nat., vol. v. 

 p. 293, Taf. y\ figs. 1, 2, 3. 



