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discoverable, the whole of it having its crystalline particles 
arranged in the same direction as the calcareous spar in the 
interior. Where, however, there w^as muddy impurity in 
the interior, the shell could readily be distinguished. More- 
over, by examining the ammonites which are agatized, I 
found that the exterior shell was very frequently distinguish- 
able from the infiltered agate, by being more transparent ; 
but this transparent part was not so thick as the shell 
originally was, and in some cases did not occur at all. The 
septa, however, being much thinner than the exterior shell, 
could not be detected in the agate. These facts, therefore, 
show that, though we cannot detect any trace of shell in 
the reniform bodies which are filled with pure agate or 
calcareous spar, we are not warranted to conclude that it 
did not at one time exist. On this account I very carefully 
examined a thin section of considerable size, in order to 
ascertain whether there existed indications of any of these 
bodies having been burst in, and whether the arrangement 
of the impurities introduced by that means afforded evidence 
of the former existence of the shell. 
Although such cases are rare, yet I have found some 
which unquestionably proved that they had been broken, 
and from the manner in which the impurities occur, I think 
that the existence of a shell is rendered very probable. In 
one instance the section of an agatized reniform body showed 
that it had been ruptured at one end, and the impurities 
introduced by that means into the interior are collected in 
such a manner as clearly to indicate the existence of a shell ; 
a distinct uniform line or space of clear agate occurring 
between the outer boundary of the reniform body and the 
extraneous granular matter that partially occupies its in- 
terior. In another instance, a section of a calcareous body 
exhibits a thin layer of fine granular matter following the 
curvature of the inner surface of the reniform body, and 
lying within, and at a slight uniform distance from its outer 
