384 SUPPLEMENT 
anomalous body, continues and determines this part of the 
mantle to deposit more nacreous matter, from which results 
a tubercle more and more thick, and even more and more re- 
gular. The original inequalities being necessarily effaced 
more and more in proportion as the new layers are deposited, 
the consequence is the production of a pearly mass ; but, that 
this should form a pearl, properly so called, that is, that the 
mass should assume a form more or less regular, either globu- 
lar, oval, or pyriform, is a matter entirely owing to chance. 
Moreover, in this species of pearl, it is impossible to conceive 
that there should not always remain, at some part of the pearl, 
a pedicle more or less narrow, in proportion to the bigness 
of the swelling, and which is the place where the disturbing 
cause commences to act. A pearl of this kind cannot, there- 
fore, by any means, be compared to an animal or vegetable 
excrescence ; for in the latter, the growth, the augmentation, 
takes place within, whereas the contrary is the case with the 
pearls. 
Another genus of pearls is that, the observation of which we 
owe to Redi. On opening many pearls, he has:constantly found 
in their interior a foreign body, like a little grain of sand, and 
in this case the formation is very easily conceived. Neverthe- 
less, considering that itis only the mantle itself which produces 
the coquillaceous matter, nacreous or not, we must suppose 
that this grain of sand, which has penetrated by accident into 
the interior of the shell, being a foreign body, and in relation 
with a part of the mantle, has produced a point of irritation, 
and subsequently a continual deposition of layers of nacreous 
matter, somewhat like calculi in the bladder. Then we per- 
ceive why, in this genus of pearls, there should be no pedicle 
of insertion. They must be equally smooth in their whole 
circumference, and it is more conceivable why they should be 
spherical. As to their form, bulk, and even the beauty of their 
nacre, (water, as it is sometimes termed) these are matters 
