4 INTEODUCTION. 



water, all the members of the class live in the sea. It is interesting to note that the 

 spicules of a fossil species of Spongilla have been discovered in chert, of freshwater 

 origin, in beds of Purbeck age. 



The Alterations produced hij Fossilization in the Structure of Sponges. — The 

 changes to which fossil sponges have been subjected in the process of fossilization 

 have oftentimes resulted in the partial or complete destruction of their original 

 spicular structure and the replacement of the mineral composition of the skeleton to 

 such an extent that great controversy has arisen respecting its original nature. In 

 recent sponges the skeleton is composed either of amorphous silica or carbonate of 

 lime with a small quantity of organic material. In fossil sponges the organic material 

 would of course disappear, and only the mineral constituents of the spicules remain. 

 But even the mineral portion is seldom, if ever, in the same condition as in recent 

 spicules ; the amorphous silica and calcite have been replaced by crystalline silica 

 and crystalline calcite, as well as by peroxide of iron and iron pyrites ; whilst not 

 infrequently the entire mineral structure has been dissolved and removed, leaving the 

 empty moulds of the spicular skeleton in the matrix. As a result of these changes, 

 siliceous sponges now occur with skeletons of calcite, and calcareous sponges with 

 fibres composed of silica. These changes are intimately connected with the character 

 of the strata in which the sponges are imbedded, but the causes producing them have 

 not up to tlie present been satisfactorily determined. In general the sponges in 

 calcareous strata have undergone the greatest alteration, the siliceous structures 

 being replaced either by calcite or iron peroxide, or dissolved away altogether, whilst 

 the structures of calcareous sponges, in common with the shells of mollusks in the 

 ^ame strata, are oftentimes replaced by silica. In arenaceous or glauconitic strata, on 

 the other hand, the changes, whether of siliceous or calcareous sponges, have been 

 much less extensive than in strata of a calcareous character. 



Changes produced in the Structure of Siliceous Sponges. — It may be affirmed that 

 no examples are known in which the siliceous spicules or fibres of fossil sponges retain 

 the perfect amorphous condition of the silica as in existing sponges. The nearest 

 approach to this original amorphous condition is shown in sponges preserved in the 

 glauconitic marls of Westphalia of the age of the Upper Chalk. The spicules of 

 these sponges have a smooth milky white aspect, and, when examined under the 

 microscope mounted in Canada balsam, exhibit the details of the interior canals and 

 the spinous adornment of their surfaces in beautiful preservation. When viewed by 

 polarized light they are either neutral or occasionally present faint prismatic tints. 

 The main feature which distinguishes these from recent spicules is their white porcel- 

 lanic aspect, which contrasts greatly with the glassy appearance of recent spicules. 



The mineral condition of some of the siliceous sponges from the Upper Green Sand 

 of Warminster also resembles that of the examples from Westphalia mentioned above ; 

 but they present the interesting peculiarity of being inclosed in a matrix of translu- 



