THE INDIGENOUS FAUNA. 29 



reformation in the hands of Professor Zittel, in Germany, and 

 MM. Carter and Sollas, in England. 



The old system of founding genera and families upon such 

 characters as the simple or compound habit of the sponge, the 

 openness or closeness of the cup, presence of epitheca, or according 

 as the oscular openings were on the concave or convex surface, 

 has resulted in the present utter confusion of nomenclature. Such 

 characters were, indeed, often not even of specific value. 



Unfortunately the modern reformers are not yet in harmony 

 with respect to the nature of the group of sponges to which most 

 of those at Upware and Brickhill belong. All the sponges from 

 these beds (as also from Farringdon) now consist of carbonate of 

 lime. Every fibre as now found is made up of that material, and 

 Professor Zittel, having regard to this fact, and attaching especial 

 importance to the occurrence of triradiate spicules in the fibres, 

 regards them as having been likewise originally calcareous sponges, 

 as they are now, in spite of their being so different from all the 

 living representatives of the Calcispongidce. Mr Sollas, on the 

 contrary, giving more weight to the preponderance of simple 

 bacillar spicules in the fibres, argues that all the spicules were 

 originally silicious. That such a replacement of silica by car- 

 bonate of lime has often occurred in Nature, especially amongst 

 the sponges, is recognised on both sides, so that the dispute affects 

 only this particular group. In this connexion the discovery of the 

 silicious hexactinellid sponge Plocoscyphea is of especial interest, 

 for no one will contest the original flinty composition of this 

 sponge, whilst now its most delicate structure is beautifully pre- 

 served entirely in carbonate of lime. Thus Mr Sollas' theory of 

 substitution receives some special support from our Brickhill 

 fossils ; but we must leave this question to be settled as time goes 

 on and new experience is gained. 



We can however no longer rest content with the old methods 

 of classification, but must in future base our classification firstly 

 upon the nature and form of the constituent fibres and spicules, 

 using only with great caution the features of external form, in- 

 tramural canals, and surface oscula as generic characters. " The 

 microstructure of the skeleton alone is of decided importance for 

 the determination of all sponges," are the words of Professor Zittel 1 . 

 1 Translated by Mr Dallas, Ann. and Map. Nat. Hist. 1879, p. 306. 



