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



Euplectellid or not. Jt is not even certain whether the parietal apertures observed in the 

 skeleton are really open during life, or whether they are covered by the soft parts. 



Genus 5. Rhahdoi'^ectella, 0. Schmidt. 

 Only one species has yet been referred to this genus. 



Rhabdopectella tintinnus, 0. Schmidt (PI. XII. figs. 8-12). 



Among the Hexactinellida from the Bay of Mexico which have been studied by 

 Oscar Schmidt, a form occurs represented by several fragments and by one whole young 

 specimen, in which the cup-shape manifest in the young form at least, the method in 

 which the spicules are united by fusion and synapticula in the basal part, but 

 more loosely in the upper portion, and further the form of the loose spicules present in 

 the soft parts, justify its reference to the Euplectellidae, though we do not know either 

 the form or structure of the entire adult sponge. The inferior portion of the mature 

 Rhabdopectella tintinnus, as figured by 0. Schmidt,^ presents so firm and stalk-like a 

 mass that in spite of the muddy character of the ground in question, I am far from 

 accepting the opinion of Oscar Schmidt, that a root-tuft must have been present, for the 

 expanded basal plate of the young specimen by no means excludes the supposition that 

 the sponge was fixed to some hard body. 



The wide-meshed lattice-like framework of the cup-shaped bod}', composed as it is of 

 greatly prolonged hexacts and numerous diacts, may well be compared with that of Rega- 

 drella and other firmly sessile EuplectelUdse. Of the looser spicules Oscar Schmidt has 

 carefully described and figured several noteworthy forms. Among these the floricomes, which 

 agree throughout with Euplectella floricomes, deserve special attention ; further, there are 

 discohexasters of different kinds — first, those with small transversely disposed stellate plates 

 on the thin extremities of the long secondary rays, of which two or three are attached to 

 every principal ray ; secondly, those with hemispherical, marginally toothed terminal umbels 

 borne by the thin, but externally conically thickened terminal rays, of which four occur 

 on each principal, and in which the S-shaped curvature (PI. XII. fig. 8) produces a mutu;il 

 entanglement; thirdly, a form resembling the latter but with long, parallel, marginal 

 prickles which run back from the hemispherical terminal disc, close to the axis of the 

 terminal ray ; and finally, a very small, on the whole spherical form in which each of the 

 long, thick, principal rays bears five strong terminals. These five terminals are arranged 

 in a regular manner, so that a somewhat shorter straight ray forms the direct extension 

 of the principal, while the four other longer and slightly bent rays are disposed in a 

 cross, and run obliquely outwards. Every terminal ray is provided with a hemispherical 



^ Loc. cit., pi. viii. fig. 9. 



