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



species appears to have a distiuct affinity with Trichostemma ; its small size, general form, 

 and projecting spicules seem to indicate this ; for the same reason it stands well apart 

 from other species in its own genus. 



Locality.— Station 246, July 2, 1875 ; lat. 36° 10' N., long. 178° 0' E.; North Pacific 

 Ocean; dejith, 2050 fathoms; bottom, Globigerina ooze; bottom temperature, 35°-l. 

 One specimen. 



Genus Polymastia, Bowerbank (Pis. XLL, XLIL, XLIV.). 



1864. Pdhjmastia, Bowerbank, Mon. Brit. Spong., vol. i. p. 177. 



1870. {?) Rinalda, Schmidt, Spong. Atlant. Gehiet., p. 51. 



1885. Wcherolla, Vosmaer, Sponges of " Willem Barents" Expedition, 1880-81, p. 16. 



Suberitid^ of massive, sessile form, with more or less numerous mammiform processes 

 on the upper surface, some of which may bear oscula at their summits, but usually 

 without visible openings. Skeleton radiately arranged ; stout main fibres run vertically 

 to the surface, and there is a cortical layer of smaller spicules arranged vertically to 

 the surface, extending both over the body and the mammiform processes. Megasclera tylo- 

 styli or styli. Sponge usually attached and without any supporting fringe of spicules. 



Vosmaer {loc. cit.) gives the following diagnosis of his genus Weberella : — " Massive, 

 globular, very compact. The surface covered with stout, short papillae. On sections 

 a rind is nearly always distinguished from the mark. Main canals surrounded by 

 compact connective tissue. Canal system of the fourth type. Spicules acuate or pin- 

 shaped." The chief difi'erence from Polymastia appears to lie in the shortness of the 

 mammiform processes, but here, as also in Polymastia, they are not merely slightly 

 elevated oscula but sometimes closed at the extremities, hence we can see no reason for 

 separating the two genera. 



We have some reasons for believing that the genus Trichostemma will also ultimately 

 have to be united with Polymastia,, but further investigation than the time at our 

 disposal will allow of is necessary before taking this step.^ 



The genus Polymastia is chiefly confined to deep water, but curiously few repre- 

 sentatives of it were obtained by the Challenger. 



Polymastia rohusta, Bowerbank (PI. XLL fig. 8). 



1861. Eu2^lectella rohusta, Bowerlsank, List of British Marine Invertebrate Fauna, Rep. of Brit. 



Assoc, for 1860, p. 236. 

 1866. Polymastia rohusta, Bowerbank, Mon. Brit. Spong., vol. ii. p. 62; see also op. n't, 



vol. iii., pi. X., p. 23, and vol. iv. p. 31. 



This species is represented in the collection by a single fine specimen (PI. XLL 

 fig. 8) dredged off the south of Nova Scotia ; in external appearance the specimen has a 



1 Cf. footnote, p. 218. 



