. ,f 2 __ 



with transmitted light, the surface of the spicules appears to 

 be rough, and pitted all over with small depressions, which 

 in some examples are so strongly marked as to give a retic- 

 ulated aspect to the surface. In the larger spicules the erosion 

 which has produced the pitted surface has not affected the 

 form of the spiculc, but in the smaller and more delicate spi- 

 cules , it has almost eaten through the spicule and rendered 

 it, particularly near the extremities, very rough and jagged. 

 As will be readily understood, the delicate spines with which 

 the spicules of both fossil and existing sponges are frequently 

 adorned, would be the first to be destroyed by the erosive 

 action and still readier would the minute flesh-spicules be dis- 

 solved by it, so that no expectation could be entertained ot 

 discovering these microscopic bodies in the material. The 

 interior of the spicules shows numerous minute ill-defined 

 brownish spots from which , as from so many centres, ex- 

 tremely small radial fibres extend in all directions. When 

 examined by polarised light these fibrous rays display pris- 

 matic tints similar to those peculiar to chalcedony, thus showing 

 that the silica in these spicules, has been changed from the 

 colloidal condition of the mineral to the crypto-crystalline state 

 of chalcedony. In the brownish spots in these spicules the 

 prismatic tints are far less clearly displayed than in the fibrous 

 portions. There arc however other spicules in the material, 

 belonging to various genera of sponges, which, instead of the 

 dull glassy aspect, common to the great majority of these 

 sponge remains, exhibit, by reflected light, a white snowy ap- 

 pearance, and when these snowy-white spicules are mounted 

 in Canada balsam they display a thin outer coating of silica, 

 which is transparent and also gives prismatic tints in the same 

 manner as the ordinary spicules; within this, however, the 

 silica is of a brownish granular aspect as if intermingled with 

 ochreous particles, and does not give the prismatic tints under 

 polarised light. The boundary between the brownish, ap- 

 parently amorphous, silica in these spicules and the outer layer 



