Mr. W. J. Sollas on the Genus Catagnia. 359 



Order RHAFHIDONEMATA (Carter). 



Family Ectyonida (Carter). 

 Subfamily Pzectronellida (Sollas). 



Family Axinellida (Carter). 

 Subfamily Cataomida (Sollas). 



There now remain for consideration the counterclaims of 

 the Calcispongia, which find an advocate in Professor Zittel, 

 with whom it is my misfortune on this sole point to find 

 myself in disagreement. The occurrence of triradiate spicules 

 in Gatagma appears, independently of its chemical composi- 

 tion, to be the only point in which it resembles theCalcispongia; 

 in the structure of the fibre, which is, after all, a far more im- 

 portant character, the difference is complete ; and no calcareous 

 sponge has yet been discovered which presents us with curved 

 and undulating uniaxial spicules like those of our sponge. 

 Moreover, while the axial canal of calcareous spicules is so 

 small as to be almost invisible, that of the multiradiate spicules 

 of Catagma is sometimes aj^parently indicated. Nor, in 

 reference to this, can it be urged that the axial canal of a 

 calcareous spicule would be likely to undergo an enlargement 

 as it became subject to fossilizing processes. In the case of a 

 siliceous sponge, it is true that on exposure to the carbonated 

 water of the sea-floor, solution sets in and soon enlarges the 

 axial canals of its spicules till they become visible. But while 

 solution equally affects calcareous spicules, it does so in a very 

 different manner: instead of the axial canal undergoing en- 

 largement, the whole substance of the spicule resolves itself 

 into a granular mass, from which the original structure is 

 quite obliterated, and in which certainly no signs of a canal 

 can be traced. Fig. 2 (p. 360), taken from a decomposing 

 spicule of a Leuconia^ will clearly show this. 



So far as the zoological argument goes, then, it may be 

 summed up thus : — 



1. (a) No known calcareous sponge possesses a reticulate 

 skeleton with definite fibres having a spicular composition. 



{h) Such a structure is exceedingly common among the 

 siliceous sponges. 



2. {a) No known calcareous sponge possesses long curved 

 and undulating uniaxial spicules. 



[b) Siliceous sponges (ex. gr. PhakeUicij Bwk.) frequently 

 contain spicules of this form. 



