OBSERVATIONS OX THE EFPLECTELLID-E GENERALLY. J.) 



mediation of the hypodermalia, especially the large pentactinic 

 hypodermalia. Amongst the Euplectellids, the liypodermalia are 

 thus far known only in the frontal lattice of Placosoma paradiclyuni 

 but in no case have hypodermal pentactins been found. Here 

 is, I think, an important negative character which distinguishes 

 the family from such others as are most liable to be confounded 

 witli it. 



After what I have said al)ove, the diasrnosis mav be made 

 so as to read somewhat as follows : 



Lyssacine Hexasterophora" of tubular, cup-like or 

 massive body; sometimes stalked; either rooted by a 

 tuft of basal spicules or firmly attached by compact 

 base; generally possessing numerous separate oscula. 

 Dermal skeleton composed of hexactinic dermal ia, the 

 proximal ray of which is as a rule much longer than 

 any other in the same spicule; no hypodermal pen- 

 tactins. Hexasters various. 



* I adopt F. E. Schxtlze's ('99, p. 93) system of dividing the Hexactinellida — the 



living Hexactinellida, at least — into two great primary groups or suborders, the Aniphidisc- 

 ophora and the Hexasterophora. Here, as elsewhere in this paper, the term "lyssacine" 

 is used without implying the Lyssacina as a systematic group, but merely to denote the 

 condition of those Hexactinellids, in which the parenchymalia are all or mostly free and 

 unfused, or in which tiiese may sometimes be extensively ankylosed and then consist not 

 exclusively of hexactins, but of hexactins and their derivative forms or of the latter only. 

 The term " dictyonine," as employed by me, refers to the state of those Hexactinellids in 

 which the parenchymalia, consisting of hexactins, and as a rule of hexactins only, undergo 

 fusion among themselves from a very early period of their existence. A brief exposition 

 of my views on the Hexactinellid phylogeny and system, incomplete and somewhat pro- 

 visional as these necessarily are in the present stage of my studies, may help to clear up 

 the points in question, and may at the same time serve to indicate the position of the 

 different families described in this Contribution. 



For the ancestral Protohexactinellida is to be assumed a lysmclne form, in which the 

 spicules consisted, mainly at least, of more or less regularly developed hexactins. Now, 

 the Amphidiscophora siiould represent a very early differentiated branch of the Hexactinellida, 

 which has remained tlioroughly lyssacine in character and has been, on that account, in a 

 position to give rise to such manifold variations of triaxonic spicules as we see in that 

 group. 



