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reniform bodies in the calcareous grit are filled is usually 
nearly pure, whilst that in the intermediate spaces contains 
much impurity. In those few cases where that in the reniform 
bodies contains a little impurity, it does not show any trace of 
concentric structure, but is in precisely the same state as that 
in the chambers of the ammonites which occur in the rock. 
Moreover, oolitic concretions are often very irrregular in their 
forms, and frequently two or more coalesce, which is the 
reverse of what is found to occur with the reniform bodies 
under consideration. I have found, by examining the oolitic 
portions of the coralline oolite, that they also exist in it, 
though in no great numbers. In it the difference between 
them and the concretions is very marked, and they form 
the nuclei of some of them (as shown by Fig. 8, which 
is one magnified 150 linear,) precisely in the same manner 
as several species of well-known forms of foraminifera and 
other small shells. At first I thought that they might be 
globular siliceous spiculae of sponges, but after finding some 
which were filled with calcareous spar, I abandoned the 
idea, for it does not appear to me at all probable that the 
siliceous matter could have been removed, and its place 
occupied by calcareous spar. 
I have already stated that these reniform bodies occur 
both filled with calcareous spar and with agate. This is 
precisely the same with the ammonites which are found in 
the rock along with them, and I, therefore, judged that 
it was probable that they were shells similarly filled with 
infiltered mineral matter. To ascertain this, it was, of 
course, necessary to determine whether they originally pos- 
sessed shells or no. I therefore carefully examined a 
section of a portion of the rock containing an ammonite 
filled with calcareous spar, in order to learn what kind of 
evidence it presented of the former existence of a shell. By 
examining it by polarised light, I found that where no im- 
purity had entered, there was no trace whatever of the shell 
