362 Mr. W. J. SoUas on the Genus Catagma. 



fossil as Caunojwra forming limestone beds several feet in 

 thickness, and no obvious deposits of silica associated with 

 them, one may feel tolerably certain that these fossils have 

 always possessed a calcareous and not a siliceous composition. 

 Now the importance of this observation is, that the Faring- 

 don and Upware sponges do not contain any trace of silica, 

 imless it enters into the composition of the yellowish, trans- 

 pai'ent, insoluble mineral which fills up the interstices of the 

 calcitic infilling of the sponge. It would be difficult to make 

 an analysis of this substance ; and I could only observe in 

 addition that it had no action on polarized light. If the fossils 

 themselves are without any signs of deposited silica, so too 

 are the surrounding strata ; no mention is made of flint, chert, 

 or silicified shells occurring in them in any descriptions which 

 I have read. This, then, would certainly be a difficulty in the 

 way of our interpretation, were it not for the exceptional cha- 

 racter of the Faringdon and Upware beds ; considering that 

 these are deposits of sometimes loose, sometimes consolidated 

 gravel, subject, in all probability, to current-action in a shal- 

 low sea during deposition, and to the free drainage of perco- 

 lating waters subsequently, one could scarcely expect to find 

 the silica from dissolved organisms retained in their immediate 

 vicinity and deposited in the same way as it is in close fine- 

 grained de])osits of chalk and limestone. 



Thus the mineralogical argument cannot, in this case, be 

 said to favour either side, and we are left to depend on struc- 

 tural character alone. This to me indicates a far closer alli- 

 ance with siliceous than calcareous sponges ; and I wait with 

 some expectancy for Oscar Schmidt's descriptions of the 

 sponges brought home by the ^ Challenger,' in the hope that 

 new forms will be found amongst them to obliterate the 

 only wide difference which now distinguishes Catagma from 

 Plectronella. 



It is not necessary to stay now to describe the outward form 

 and general characters of Catagma ; it is sufficient to refer to 

 Sharpe's paper on the Faringdon sponges, and to indicate 

 those forms which should be placed in this genus. They are 

 as follows : — 



Catagma jy^ziza (Manon, Sharpe) ; 

 C. macroporus (Manon, Sharpe) ; 

 C. porcatum (Manon, Sharpe) ; 

 C. faring donense (Manon, Sharpe). 



In the family of the Catagmida must be included Tragos 

 faringdonensis and ScypJiia foraminosa. 



I cannot conclude, however, without remarking upon the fact 

 that Professor Zittel not only assigns the fibrous sponges with 



