Carboniferous Sponges from Ayrshire. 427 



We have referred this sponge to Hyalonema ratlier than to 

 Rossella, because the rope is shorter and the spicules are 

 larger, though, on the other hand, the rods are smooth as in 

 Rossella, not spinulose as in Hyalonema. The combination 

 of characters might be less marked were we able to appropriate 

 to our species the other kinds of spicules proper to it. The 

 sponges previously found in British Palaeozoic rocks are 

 M'Coy's Silurian genus Acanthaspongia, Salter's Silurian 

 genus AmpJiispongia, and Salter's Silurian genus Proto- 

 spongia, the latter of which is placed by Zittel alongside of 

 Steganodictyum, the two former, the hrst certainly, being- 

 referred to the Lyssakina of Marshall, in which Hyalonema is 

 placed; but we do not propose to discuss their affinities so long 

 as the information at our disposal is incomplete. We regret, 

 however, that Zittel has (N. Jahrbuch, 1877, p. 371) placed 

 our species in the subgroup Monakidaj of Marshall, and thus 

 removed it from the PoUakidas, among which Rossella and 

 Hyalonema are found. While we think H. Smithii a good 

 species, we do not say it is the only one. The variety in size 

 of the sexradiate spicules is not necessarily a proof of specific 

 diversity ; it may only correspond to variety in age and size of 

 the individuals. Among the many parcels of the rotten lime- 

 stone which Mr. Smith has distributed, some may yield com- 

 binations of spicules in situ, throwing light on the specific 

 unity or diversity. 



The mode of fossilization of these objects is interesting. 

 The rope is very widely distributed throughout the Scottish 

 Limestones, and in all cases is siliceous ; but its condition 

 varies : on the one hand it is only slightly more opaque than the 

 rope of recent forms ; on the other, the siliceous material may 

 (figs. 5, 6) have deserted its proper place and be found outside 

 a circular empty tube in whose middle a siliceous rod repre- 

 sents the once empty canal. Again, the concentric lamination 

 of the recent forms may be recognizable (fig. 9) even though 

 the rod be compact, horny, and yellow; while, on the other hand, 

 a rod of botryoidal chalcedony (fig. 10) may represent the once 

 homogeneous cylinder. In fig. 1 some of the dark lines repre- 

 sent spaces from which the rods have disappeared, an irregular 

 congeries of chalcedony spheroids taking their place. Lastly, 

 another though rare modification is that by which (especially 

 in the sexradiate spicules) the siliceous matter assumes a 

 granular form. Evidence of the pressure to which the fossil 

 has been subjected is given by the longitudinal grooves which 

 adjacent rods have imprinted on each other, whether the 

 pressure resulted from compression or from expansion of the 

 silica during metamorphosis. 



