158 On a new Monaxonid Sponge. 



linear skeleton-spiciiles, one which has a long, smooth, nsuallj 

 crooked shaft, evenly rounded off at tlie base, and at the apex 

 branching into several short blunt processes, like the fangs of 

 a human tooth ; here, it appears, that we have a polyaxial 

 spicule comparable to the forked spicules of the Tetractinellida, 

 derived from the monaxonid type by furcation of the main 

 axis instead of by the outgrowth of spine-like processes. The 

 central canal appears to branch together with the spicule. 

 The systematic position of this sponge is very doubtful ; it 

 forms the type of a new genus which, owing to the presence of 

 trichite sheaves, we have included amongst the Desmacidinida^; 

 but it has very strongly marked axinellid characters and is 

 far removed from the Suberitidje. 



One of the most interesting and important sponges which 

 bears upon this question is the species described by Prof. 

 Sollas in the ' Annals and Magazine of Natural History ' for 

 January 1879 (p. 17), under the name Plectronella jjapillosa^ 

 a species which subsequently proved to be identical with 

 Tricentri'um muricatum^ Ehlers. This monaxonid sponge, 

 which the author refers to the family Ectyonida^, possesses an 

 echinating skeleton-spicule which is normally triradiate and 

 occasionally quadriradiate. " These are true quadriradiate 

 spicules, and thus seem to lead on to the tetractinellid type." 

 Carter* also refers this sponge to tlie family Ectyonida^; but 

 Vosmaerf considers it a Tetractinellid. 



All this evidence seems to lead to the conclusion that the 

 presence of a tetractinellid s])icule is in itself not a sufficient 

 guide as to the systematic position of any given sponge, that 

 it may arise independently in different groups of sponges, 

 and that the Tetractinellida are by no means so far removed 

 from the Monaxonida as is generally supposed ; indeed, Prof. 

 Sollas tells us that he began to doubt long ago how far the 

 Tetractinellida form a natural group. There seems now to 

 be no doubt tliat they are derived from monaxonid forms, but 

 whether they have originated polyphyletically or not is another 

 question ; so far as spiculation is concerned they may very 

 well have done so ; but this is not the place for a discussion 

 of this question. The new sjjonge which we have here de- 

 scribed lorms a very important link in the chain of evidence, 

 and as such seemed to be deserving of special notice. 



In conclusion we take this opportunity of thanking Prof. 

 Sollas ibr the invaluable assistance which he has given us in 

 compiling this short account. 



* Aril. & Mag. Nat. Hist. ser. 5, vol. iii. p. 203, pi. xxvii. fig. 13. 

 t Bronn's ' Klass. u. Ordnuiig. des Tliierreichs; Porifera,' p. 3i'2. 



