so-called u Far ring don Sponges. 11 31 



siliceous ; it may be easily seen that as they lie in the grey 

 compact limestone (now before me) they present themselves 

 under a smooth form, which, on being carefully extricated and 

 placed in nitric acid, dissolves away completely with strong 

 effervescence ; and on boiling a portion of this limestone so 

 charged, it is found that many of these spicules come out in 

 the form of a ragged cylinder of silex, which, in the rotten or 

 decomposed parts of this limestone, present themselves in the 

 state of chalcedony fretted out by rhomboidal cavities (No* 6, 

 pi. ix. fig. 14, a, b } c). 



Now the u mountain (Carboniferous) limestone " is analo- 

 gous in this series to the " chalk " in the Cretaceous Sys- 

 tem ; and if the double change in mineral composition has 

 taken place in the former, why may it not have done so in the 

 latter, in some although not in all instances ? Thus, why 

 might not Pharetrospongia Strahani have been siliceous in 

 the first instance, just as much as Acanthospongia Smithii and 

 Pulvillus Thomsoni? 



It is not my business here to deal with the processes of 

 transitionary mineralization and their why and wherefore (I 

 know nothing about them comparatively, anymore than of 

 mineral metamorphism in general, or the elevation and depres- 

 sion of whole continents), but to deal with facts ; while Zittel 

 himself cautions us, in these instances, against the employ- 

 ment of arguments based on chemical reasoning (No. 8, vol. iii. 

 p. 366). 



Having got so far as, in my opinion, to throw some doubt 

 over the original nature of Pharetrospongia Strahani and the 

 like being calcareous, we have now to ask ourselves, what 

 evidence there is of the existence of any fossil sponges, like 

 those of the Monactinellid series of existing ones now in the 

 Zoological Department of the British Museum, which of 

 themselves as much outnumber the existing Hexactinellida 

 and Lithistida there as the latter do the former in the fossil 

 collection of the Geological Department — indeed more so ; for 

 I do not know of a single instance where an undisputed ex- 

 ample is to be found in the latter — considering Pharetro- 

 spongia Strahani and the like spiculations calcareous. 



If we consult some of the highest authorities on the entire 

 forms, we find, as in Quenstedt's i Petrefactenkunde ' (No. 7), 

 nothing in this way but Hexactinellida, Lithistida, and the 

 so-called Calcispongias. If on their minute structure y we find 

 in Zittel's elaborate and invaluable investigations nothing 

 beyond a comparatively insignificant mention of the Monacti- 

 nellidse, under his genera Opetionella and Scoliorhaphis , toge- 

 ther with Cliona } which, being an excavating sponge, is recog- 





