— 13 — 
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 spicule, 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 ail 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 are 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 
