156 PORIFERA. 



condition. The alteration is in this case but slight, since the silica is 

 still in the colloidal state, and the principal change that has occurred is 

 that the spicules have lost their original glassy aspect, and have become 

 porcellanous and milky-white when viewed by reflected light. 



(b.) The silica of the spicules may have become cryptocrystalline or 

 crystalline. In this form of alteration, the original colloidal silica of the 

 spicules has become changed into chalcedony or quartz, and the spicules 

 now exhibit with polarised light the same colour-changes as are shown 

 by the above minerals. 



(c.) In a great many cases among fossil Sponges, the original flinty 

 skeleton has undergone more or less complete solution during fossilisa- 

 tion, the Sponge being now represented only by hollow casts of the 

 original skeleton in the matrix of the enclosing rock. In the case of 

 calcareous organisms it has long been well known to palaeontologists 

 that the skeleton very commonly undergoes solution, leaving nothing but 

 a hollow mould or impression in the rock to mark its former existence. 

 Silica being a much more stable substance than carbonate of lime, it used 

 to be supposed that an originally flinty skeleton would not be liable to 

 undergo a similar solution. It has, however, been conclusively shown 

 by Zittel, Sollas, and Hinde, that this supposed stability of organic silica 

 is largely imaginary. According to the last-named observer, " it may be 

 accepted as proved that silica in the colloid state, in which it occurs in 

 the skeleton of recent siliceous Sponges, and also in the original condi- 

 tion of fossil Sponges, is extremely liable to chemical changes, and that it 

 is only when it is in the condition of chalcedony, or is crystalline, that it 

 can be regarded as stable. The changes in the siliceous skeletons of 

 fossil Sponges, mentioned above, show the tendency of the silica to pass 

 from the unstable colloid to the stable chalcedonic or crystalline condi- 

 tion. Under favourable conditions this chemical change has taken place 

 without destroying the form of the spicular skeleton, but in other circum- 

 stances the colloid silica of the skeleton has been wholly dissolved away, 

 and redeposited, usually in the chalcedonic condition, so as to form solid 

 beds of chert and bands of nodular flints." 



(d.) In the cases just spoken of, the original siliceous skeleton of the 

 Sponge has undergone solution, and there is left in the rock a hollow 

 mould of the skeleton, this mould being commonly so accurate as to 

 preserve with fidelity the form of the component spicules of the 

 skeleton. Very usually, however, the mould thus formed becomes 

 ultimately completely filled up with some mineral substance de- 

 posited from the water which percolates through the rock ; the result 

 being the formation of a " pseudomorph " of the original Sponge, or, in 

 other words, a body which has the exact form, and may even have the 

 microscopic structure, of the original Sponge, but which consists of some 

 secondary substance which has taken the place of the original skeleton. 

 The substance which has in this way replaced the original Sponge is 

 most commonly crystalline calcite, but peroxide of iron is likewise often 

 the replacing material, or, less commonly, iron-pyrites or glauconite ; 

 while in some cases it would appear that the replacing substance has 

 been chalcedonic silica. It only needs to be added in this connection 

 that the changes above spoken of do not necessarily affect an entire 

 Sponge, but that specimens often occur in which a portion of the 

 skeleton has been fossilised in one way, while another part may have 

 been preserved in a different manner. 



The CalcispongicB do not, as a rule, undergo fundamental alteration, 

 as regards their skeletal structures, during fossilisation. In some late 



