ME. A. DENDY ON WEST-INDIAN CHALININE SPONGES. 351 



The first of these two general laAvs is very clearly demonstrated in the case of 

 SpinoseUa sororia, D. & M., sp., of which I distinguish three varieties in addition to 

 the typical form (PL LIX. figs. 1, 3 ; PI. LXIII. figs. 1, 2), and in the case of Paclnj- 

 chalina variabilis, mihi (PI. LX. fig. 2) ; but as this will be sufficiently clear from the 

 figures and from the descriptions given in the systematic portion of this paper, I shall 

 not consider the question any further in this place. 



The second law has already been very strongly insisted upon by Mr. Eidley and 

 myself in our Report on the Monaxonida collected by H.M.S. ' Challenger,' in which 

 we have endeavoured to show that the so-called " Keratosa " have probably descended, 

 polyphyletically, from several distinct groups of siliceous sponges, amongst which the 

 Chalininae figure prominently. The view that the Keratosa have been derived from 

 siliceous sponges has been gradually gaining favour with naturalists for some time past, 

 and is now, I belie\e, very generally admitted to be con-ect. But I have nowhere 

 found quite such strong arguments in its favour as amongst the ^Yest-Indian Chalininae ; 

 for here we can trace in different species of the same genus the gradual degeneration 

 and disappearance of the spicules until we come down to forms like Sj)inosella 

 maxima, mihi (PI. LXI.), and Sjji7ioseUa plicifera, D. & M..(P1. LVIII. fig. 5; 

 PI. LX. fig. 1), which sometimes still contain traces of the spicules imbedded in the 

 horny fibre, and apparently on the verge of disappearance, while at other times they 

 contain no spicules whatever; and yet the specimens with spicules and those without 

 are specifically indistinguishable. 



It appears that the spicules may persist as vestigial structures long after they have 

 ceased to be of any functional importance, and that they disappear first from the 

 secondary skeleton-fibres. Thus in the genus Siphonochalina we have S. spiculosa, 

 mihi (PI. LVIII. figs. 2, 2a; PI. LXIl. fig. 8), with great numbers of well-developed 

 spicules constituting a most important part of the skeleton-fibre, and, on the other 

 hand, S. ceratosa, mihi (PI. LVIII. figs. 1, 1 « ; PI. LXII. fig. 2), in which the spicules 

 have almost completely vanished ; while 8ip]tonochalina procitmbens, Carter (PI. LVIII. 

 fig. 4 ; PI. LXIL fig. 1), occupies an intermediate position in this respect, containing 

 several series of fair-sized spicules in the primary fibres and only a sparse single series 

 of similar spicules in the secondaries. » 



The classificatory difficulties to which this state of things leads are obvious. It is, 

 in fact, no longer possible to draw a sharp line of distinction between the ChalininiE 

 and the so-called Keratosa ; for different specimens of one and the same species may or 

 may not contain spicules, while at the same time it is probable that a large proportion 

 of the Keratosa have no near connection with the Chalininae at all, but are descended 

 from quite different groups of siliceous sponges ^. 



Some of these difficulties are well illustrated by a note at the end of Professor 



' Cf. Ridley and Dendy, Eeport on the Monaxonida collected by H.M.S. ' Challenger,' p. Iv et se^. 



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