INTEODUCTION. 9 



ill some cases so slightly altered that the minute component spicules can be detached. 

 The fibre of the calcareous sponges from Essen resembles in appearance that of the 

 Warminster sponges ; but the individual spicules are, as a rule, not recognizable. 

 The fibre of the calcareous sponges from the Upper Chalk of the south of England 

 is in appearance smooth, white, and unaltered ; but in thin sections the component 

 spicules are rarely visible, and the fibre presents either a homogeneous mass of 

 amorphous calcite or a very fine radiate crystalline structure. 



Calcareous Sponges replaced ly Silica. — The instances in which this change has 

 been effected are comparatively rare, and limited, with few exceptions, to sponges 

 imbedded in strata in which the remains of other calcareous organisms have likewise 

 been replaced by silica. I have not met Avith a single example of this change in any 

 of the numerous calcareous sponges from the Lower and Upper Green Sands of this 

 country ; but in tlie Jurassic strata of certain localities in Germany many of these 

 sponges have become silicified, and can be perfectly freed from the calcitic matrix 

 by acid. The fibre of such sponges after treatment with acid has a rough appear- 

 ance, and is of a snowy white tint ; and, as may be expected, all traces of spicular 

 structure have disappeared. A partial replacement by silica is sometimes present in 

 examples of the well-known Pharetrospongia Strahani, Sollas, from the Upper Green 

 Sand of Cambridge. The fibre of these sponges is calcareous in the central portion, 

 whilst the exterior has been replaced by silica. This change is more remarkable 

 from the fact that the form of the individual spicules has not been destroyed, but 

 they can be distinguished in a siliceous condition. This same species of sponge 

 remains completely calcareous when imbedded in the Upper Chalk ; but when in- 

 closed in solid flints the outer surface of the fibre has become silicified, whilst the 

 interior remains calcareous, in the same manner as in the Green-Sand specimens. 



Geological Distkibution. 



Fossil sponges make their appearance in some of the earliest fossiliferous rocks, 

 and they are present, though by no means in continuous sequence, in all the great 

 divisions of the geological scale to the Tertiary era. They form, however, but a very 

 subordinate part of the fossil fauna of the Palaeozoic and early Mesozoic groups, and 

 it is not until towards the close of the Jurassic and during the Cretaceous periods 

 that they occur in sufficient numbers to constitute a relatively important feature of 

 the fauna of these rocks. Judging by the constant association of sponges with beds 

 of flint and chert in the Mesozoic rocks, there is great reason to suppose that they 

 may have existed in strata of Palaeozoic age where beds of these materials are present, 

 though up to the present they have not been recognized in them. 



Cambrian System. — The earliest known sponge is the Protospongiafenestralis, Salter, 

 belonging to the Hexactinellidse. It is only known from fragments and detached 

 spicules, preserved in black shales in strata of Menevian age in Wales and in Sweden. 



c 



