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



1869 (loc. cit.), though his generic diagnosis does not appear till 1872, and that Schmidt's 

 diagnosis of the genus Radiella is highly unsatisfactory, we think ourselves fully justified 

 in retaining Sars' generic name, although the two seem to be probably synonymous. Upon 

 this latter point, however, we should be very doubtful were it not for Hansen's precise state- 

 ment, for the absence of the upper cortical layer of spicules in Radiella would be a very 

 strong point. 



Another sponge with almost identical external form is Bowerbank's Halicnemia patera,^ 

 Ijut the genus Halicnemia is at once differentiated from Tricliostemma by the presence of 

 peculiar spicules in the dermal membrane, which Bowerbank tells us are "very numerous, dis- 

 persed, fusiformi-acerate, entirely spin ed, subangulated, and frequently infl ated at the middle. " 



Probably the peculiar external form of these sponges is comparable to the " Crinorhiza " 

 form found amongst the Desmacidonidse, and may exist in very different genera when 

 living under similar conditions ; another instance being the little known Xenospongia 

 2oatelliformis, Gray, which only wants the marginal spicular fringe to complete the 

 external resemblance, but has a totally distinct spiculation (including stellate forms).- 



Tricliostemma sarsii, Eidley and Dendy (PI. XLIII. figs. 1, la, 2, 2a, 3, 3o). 



1 886. Tricliostemma Sarsii, Eidley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, vol. xviii.p. 488. 



Sponge (PI. XLIII. figs. 1, la, 2, 2a, 3, 3a,) discoid, much flattened and usually 

 thin. Flattened more especially on one side, which bears an oscular tube in its centre, 

 and must therefore be regarded as the upper one. The lower side appears to be always 

 more or less convex but usually only slightly so ; in the single specimen from Station 

 184, however, it is strongly convex. The margin of the disk is thin, and provided with 

 a beautiful fringe of very long, silky-looking spicules. The largest specimen is about 

 13 mm. in diameter (exclusive of the fringe of spicules, which is 3 mm. broad), and only 

 just over 3 mm. in thickness at the centre. The smallest specimen has a diameter of 

 about 5 mm. (exclusive of the fringe of spicules, which is again about 3 mm. broad). 

 The diameter of the Australian specimen is about 8 mm. (excluding fringe), and the 

 thickness in the centre 4 mm. On holding a specimen up to the light and looking at 

 the flat side one sees a number of small, round, translucent patches, about 1 '5 mm. in 

 diameter, and arranged for the most part in a circle, a short distance within the margin 

 of the sponge {cf. PI. XLIII. fig. 2, &c.). The nature of these is very doubtful ; we were 

 at first inclined to regard them as pore-areas, but there is hardly sufficient evidence to 



1 Mon. Brit. Spong., vol. i. p. 184, pi. x. figs. 228-233, pi. xxsii. figs. 363, 364 ; vol. ii. p. 96 ; vol. iii., pi. xv. 

 figs. 31, 32 ; vol. iv. p. 45. 



2 Concerning the relations of Tricliostemma to Polymastia we prefer at present to keep silence. Vosmaer, however 

 (Sponges of the " Willem Barents " Expedition 1880-81, p. 12), makes the two genera identical, but, though there is 

 much to be said in favour of this view, we are, as already indicated, not yet satisfied about the matter. We cannot, how- 

 ever, at all understand how Vosmaer {loc. cit.) has come to the conclusion " that there is no generic difference between 

 Halicnemia and Polymastia," and also, of course, Tricliostemma, in spite of the difl'erences in spiciilation ; a conclusion 

 at which also von Marenzeller had previously arrived {Denkschr. d. k. Akad. Wiss. JVien, Bd. xxxv. p. 371). 



