770 NEW GENERA OF MONAXONID SPONGES, .. 



commonly termed — are phylogenetiealiy widely distinct from 

 the dermal styli,— the former undoubtedly having been derived 

 from acanthostyli* cognate with those echinating the fibres, 

 while the latter are just as certainly homologous with the megas- 

 cleres which function as dermal spicules throughout the majority 

 of Desmacidonid genera, and which I (6, p. 137) have termed 

 auxiliary megascleres. In Tenacia, on the other hand, and in 

 the closely related genus Tenaciella described below, the three 

 sorts of smooth styli which are present are not perfectly distinct 

 in kind, but are connected by an uninterrupted series of spicules 

 of intermediate size and form, thus clearly revealing their 

 common derivation from an originally single type of styli. 

 Whether it is from principal styli they are derived, however, 

 or whether from auxiliary, is not clear. Their curved and 

 slightly tapering form inclines rather to that which is character- 

 istic, in general, of principal megascleres; but also they are 

 provided basally with a cap of spinules, and in this respect 

 they display a feature which, in every other instance of its 

 occurrence, appears to be exclusively a peculiarity of auxiliary 

 megascleres. In the case of Tenacia, the analogy in spiculation 

 which it bears to various other related genera, as provided by 

 the fact that the styli occurring within the skeletal fibres are 

 destitute of basal spinules,** led me originally to consider these 

 intrafibral styli as homologous with the similarly situated prin- 

 cipal styli of Clathria. The evidence afforded by Tenaciella 

 canaliculata, however, — in which species the transition of the 

 several forms of smooth styli one into the other is gradual and 



* I have elsewhere already suggested (7, p. 454) that evidence is not 

 lacking which points to the possibility that the acanthostyli of the Des- 

 macidonidse originated from spirasters or forms related thereto. The 

 opinion to which I am inclined to subscribe is that the Desmacidonidse 

 have evolved from ancestors not remotely related to certain genera at 

 present included in the family Spirastrellidse ; and that the classificatory 

 distinction between Tetraxonida and Monaxonida, as originally pro- 

 posed by Sollas (20) is fundamentally correct. From this point of view 

 the principal and the echinating megascleres of the Desmacidonidse are 

 primitively derived from spicules homologous with the microscleres of 

 the Spirastrellidse, while in all probability their , dermal megascleres are 

 derived from spicules homologous with the tylostyli of the same family. 



* * I have, however, recorded the occasional occurrence of basal spinules 

 in the case of some of the intrafibral spicules of Tenacia paucispina var. 

 multipora (6, p. 185). 



