Dr. R. B. Soil — On Fossil Sponges. 313 



always solid and resisting, like that of the recent Dactylocalyx, 

 Stutch., the Farrea of Bowerb., etc. Many of the siliceous sponges 

 no doubt were so ; but as regards the calcareous ones it may be 

 observed that among recent species, according to Bowerbank, 

 " Carbonate of lime, as an element of the skeleton, is known only 

 in the form of spicula." ' In some cases, as will be shown hereafter, 

 the j&bres or twigs were originally complex, formed of bundles of 

 spicula, like the twigs in many of the recent species, and were after- 

 wards consolidated more or less completely by the fossilizing process. 

 But there is no evidence that in others it may not have been keratose, 

 either with or without the accessory spicula. Some of ^our recent 

 homy sponges are not less resistant than the solid siliceous fibrous 

 species, and are certainly less friable. In the fossil, however, the 

 horny fibre is replaced by silica, lime, or iron. That the tissue in 

 the fossil is not identical with that of the original sponge may be 

 inferred from the circumstance that we commonly find all the sponges 

 from one locality, or one deposit, in the same mineral condition. Thus, 

 all those from the Carboniferous Limestone of the Great Orme's Head 

 are silicified ; but so also are the associated Zoophytes, Conchifera, 

 and Gasteropods, etc. All the sponges from the Farringdon Green-sand 

 are calcareous, while those of Warminster are all siliceous. The 

 sponges of the English Oolite are all calcareous : those of the Chalk 

 are either silicified, or else in the state of moulds or casts, the walls 

 of which are stained with peroxide of iron.'^ 



It has been thought that the iron-staining of the moulds and their 

 refilling with pyrites renders it probable that in the original sponge 

 the fibre was keratose. That in the horny sponges, pyritous casts may 

 be more frequent than in the others is highly probable, but the 

 amount of sulphur in the keratose is far too small to enter into com- 

 bination with all the iron in the cast in accordance with the theory 

 implied ; and assuming that the original skeleton of the sponge was 

 keratose, there is no reason to suppose that the mould would not be 

 refilled in harmony with a general law, i.e., the cast was siliceous 

 when deposited from water holding in solution silica rendered so- 

 luble by the presence of lime and alkalies ; calcareous from waters 

 holding lime in solution by the aid of an excess of carbonic acid ; 

 iron in other cases, and even bisulphuret of lead has been found 

 replacing carbonaceous matter in the plant remains of the Lias of 

 Dunraven.^ The manner in which the mould is refilled with silica 

 was precisely similar to that by which it is made to replace the car- 

 bonate of lirae in the tests of the Mountain limestone mollusca of 

 the Great Orme's Head, the Portland rocks of Tisbury, or the Green- 

 sand of Blackdown. The Ostreidas and Serpulge, and other parasites 



1 I. c. p. 154. 



2 Both Ischadites and Ptylospongia occurred to Eichwald sometimes calcified, and 

 sometimes converted into bisulphuret of iron, more or less peroxidized. His Ma'non 

 de/orme, from Gherikoff, was silicified, while the examples of the same species from 

 the environs of Poulkowa were all calcified. Lethcea Rossica, p. 339. Ischadites 

 Kcenigii occurs in our British Upper Silurian rocks, both as a calcareous and as a 

 pyritized fossil. 



3 De la Beche, Mem, Geol. Surv., vol. i., p. 273. 



