382 SUPPLEMENT 
Passing over the intervening subdivisions as affording little 
or nothing of popular interest, we come to the genus AVICULA, 
a species of which the mytilus margaritiferus of Linnzus is 
celebrated as principally producing the fine “ Orient pearl.” 
A few short details on this subject cannot fail to be interesting 
to our readers. 
The pearl is a body of a variable volume, and of different 
forms, composed of layers of the nacreous substance, ex- 
tremely numerous and compact, which constitute a more or 
less considerable part of certain species of univalve, or bivalve 
shells, and which appears to be constantly accidental, and is 
supposed to be caused by a malady of the animal or its shell. 
This, however, is far from being an ascertained fact, as, were 
it so, the disease must extend to the far greater majority of 
individuals, so much so indeed, that every one is found to be 
accompanied by a certain proportion of minute particles, 
which are evidently the pearl in the first stages of formation; 
hence it may be fairly supposed that they are in some essential 
degree useful rather than prejudicial to the inhabitant of the 
shell. 
When we treated of the structure of the coquillaceous en- 
velope of the mollusca, we observed how it is produced and 
thickened by little and little, showing that the whole surface 
of the skin, which clothes the body, properly so called, is 
exhaled excessively from thin strata of calcareous molecules, 
dissolved in an animal mucus, and applied one within another, 
always outedging a little, from which results the augmenta- 
tion of the shell, not only in thickness, but in breadth and 
length. We have likewise seen that the necessary modifica- 
tion in this formation, to produce the nacre, or mother-of- 
pearl, seems to consist in the calcareous molecules being placed 
so as to leave very small spaces between them, in which the 
light is decomposed before it is reflected back to us. Thus 
the pearls are an animal production, in which chemists re- 
ee 
