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



Skeleton. — There is no special dermal skeleton, but spicules project in irregular, 

 sparse tufts from the uppermost portion of the main skeleton, and serve to support the 

 dermal membrane, commonly projecting slightly l)eyond it ; these tufts are nothing but 

 the projecting ends of the primary skeleton fibres. The main skeleton (in the lobate 

 specimen) is a very well-developed reticulation of very stout, compact spiculo-fibre, in 

 which there is a well-marked distinction between primary and secondary fibres crossing 

 one another at right angles and running vertically to, and parallel with, the surface of 

 the sponge. The rectangular meshes between the fibres vary much in size, usually they 

 are large. In the repent specimens, the skeleton is not so well developed, being laxer 

 and more confused. 



Spicules. — Slightly curved oxea (PI. II. fig. 10), fairly sharply and fairly gradually 

 pointed, size about 0-225 by 0-016 mm. 



The above description is taken from a series of specimens, aU from Station 142, 

 which must be considered as the types. There is also in the collection a single small 

 piece from Station 150, without oscula and j)robably young, which we identify with the 

 species ; it diflFers from the types in having a very compact skeleton with very indistinct 

 fibre, and in having the spicules rather larger, measuring about 0-3 by 0-016 mm. 



The specimen obtained from Kerguelen by the Transit of Venus Expedition, and 

 described by Mr. Carter (loc. cit. supra), must also, having regard to the locality and 

 measurements of the spicules, be referred to this species rather than to Tlmlysias 

 suhtriangularis. Here again the skeleton reticulation is closer, but the fibre not so 

 well developed as in the types of Petrosia similis ; the spicules average about 0-19 

 by 0-0126 mm. in size. 



The synonymy of the diflPerent species of Petrosia is very difficult to unravel. 

 Undoubtedly this species comes near to Tlialysias subtriangularis, Duchassaing ^ ; the 

 difficulty is to find out exactly what that species is. Even supposiug that Schmidt 

 had not examined Duchassaing's type specimens, yet, having regard to the localities, it 

 is probable that his identification of his Schmidtia aulopora'^ with Duchassaing's 

 Thalysias suhtriangularis is correct. Thalysias repens, Carter,^ also from the West 

 Indies, which Mr. Carter caUs " a repent form of the white species sithtriaiigularis, viz., 

 Tlmlysias repens, mihi," appears from the measurement of his figure of the spicule 

 (which is thus shown to be about 0-18 mm. long.) to be the same species as Schmidt's 

 aulopora. In Schmidtia aulopora, Schmidt, the spicules measure about 0-175 by 

 0-0078 mm., being thus shorter and not half so thick as in the types of Petrosia similis.* 



1 Animaux radiaires des AntiUes, p. 26. See also Duchassaing and Michelotti, Spongiaii-es de la mer Caraibe, 

 Haarlem, 1864, p. 85, pi. xvii. fig. 1, pi. xviii. fig. 1. 



^ Spong. Atlant. Gebiet., 1870, p. 44, Taf. v. fig. 8. 



3 Ann. and Mag. Nat. Hist, April 1882, p. 282, pi. xi. fig. 10. 



* The measurements of the spicules in Schmidtia aulopora are taken from a preparation in the British Museum, 

 labelled in Schmidt's handwriting, from St. Thomas. 



