Petrifactions found in St. Peter’s Mountain. 449 
teeth, but in different fockets. The grinders are not all re- 
newed, but in general three when there are fix, and two when 
th sre are five. Nature, however, is not always uniform in this 
operation. Mr. John Hunter, a worthy Member of our 
Society, has given a very interefting and complete natural 
hiftory of the teeth, in which thefe obfervations are ftated. 
In the crocodile the fucceedingor fecondary teeth appear even 
when the animal’s head is equal to two feet ; that is, when it 
has acquired one-third of its ufual growth* When they grow 
too faft, before the temporary tooth is fhed, they perforate the 
fide of the bone, at the part where they meet with the leaf! 
refiftance. Infiances of this variety occur in the large croco- 
dile’s head, which is in my collection. 
In all quadrupeds the enamel is, of the folid parts of the 
teeth, the firft formed, making a cavity, in which the other 
bony fubftance is depofited, and formed by lamellae placed 
one within another, as is obferved by Mr. John Hunter in 
the work already mentioned, p. 92. To this the root is added, 
which is filled in the fame manner till the tooth is long enough 
to pierce through the gums. 
But in the fofiil jaw-bones of St. Peter’s mountain, a 
fmall fecondary tooth is formed, with its enamel and folid root 
at once, within the bony fubftance of the primordial or tem- 
porary tooth itfelf, as is to be feen in the fmall fragment now 
in the Britilh Mufeum, and in tab. XVI. A, B, C, D, E ; which, 
by continuing to grow, feem to make by degrees fufficient ca- 
vities in the bony roots of the primary teeth : but what be- 
comes of them at laft, and how they are filed, I am not able 
to guefs. I have one in my collection, where the fucceeding 
tooth is intirely formed within the center and fubftance of the 
primordial tooth. In the 6th figure (tab.XV.) a little oval cavity is 
obfervablcy 
