ON ACEPHALA. 383 
cognize a great quantity of carbonate of lime united to an 
animal matter or mucus. They are capable of being dissolved 
in all acids stronger than carbonic acid. ‘They are also 
necessarily formed of strata, which envelop one the other, 
at least when they are perfect. It has been suggested, 
moreover, that they always contain a little foreign body, 
around which the layers are deposited. Athenzus would 
have it that they are produced in the body of the animal, and 
compares this production to that of the hydatids in the flesh 
of a leprous pig. We certainly cannot admit the opinion of 
Pliny and Dioscorides, that they are a production of dew ; nor 
without hesitation, that of Valentine, who thinks that they 
are the eggs of the females, an hypothesis, however, much 
more probable than that of Pliny. 
In carefully studying a great number of shells, the interior 
of which is nacreous, we can easily see that at the places 
where the movements are most irregular, for instance, in the 
places where the fibres of the muscles of attachment are in- 
serted in a univalve or bivalve, the nacreous substance is 
much less smooth, than at the places where the mantle only 
executes its ordinary movements of retraction and extension. 
Sometimes they even form pretty well marked swellings, and 
even sorts of irregular tubercles, as is very obvious in the 
haliotides. When a shell has received an external shock, 
considerable enough to cause it to lose substance, or even 
merely to occasion a depression of no great depth, we find in 
its interior that the nacreous matter, in depositing at first, has 
followed the inflexion produced by the depression, and after- 
wards was necessarily accumulated in this place in a greater 
quantity than if there had been no iritation, so as to form an 
irregular tubercle, more or less considerable. ‘This is evi- 
dently the origin of at least one genus of pearls; for, once 
that the parallelism is lost in the deposition of the forming 
layers, the irritation produced by this sort of foreign or 
