PE.VULS-- -THEIR FHRMATICIN. 
:v2i 
separate m- attaclied calcareous cencretioiis, which are especially 
characteristic of the Pelecypods, and being formed by the same or 
similar glands to those secreting the shell resemble it in character ; 
the concretions of ^[rc<( are nsnally violet, of Phnid rose coloured, 
and in other groups they likewise appertain in colour and structure 
to the interior of the particular shell to which they belong, the 
species with porcellanons shells producing concretions resembling 
marble, while those from nacreous shells display a more or less 
brilliant opalescent lustre and are distinguished as Pearls. 
Pearls are perhaps the most valuable production of the mollusca, 
and, hehu'C the systematic and successful prosecution of the Oriental 
pearl-tisheries, were very largely derived from the Unin nuu'garitifer, 
a species which e.xists not only in the British Isles, hut throughout 
the northern circumpolar regions, the production of pearls being 
formerly so especially identitied with the Unionida', that the name 
Cnio, signifying a pearl, was ap])lied to the tyiiical genus. 
Pearls, as well as the kindred but less beautiful porcellanons or 
tibrous concretions, consist of successive and alternating la3mrs of 
calcareous ami chitinous matters, deposited around any irritating ex- 
traneous particle or particles which may have gained entiy amongst 
the delicate glandular tissues. In some localities the eggs or bodies 
of Atd.r, lhirtg)lidliis, or other of the minute internal parasites may 
he the predisposing cause, in others a grain of sand or other foreign 
irritant substance may furnish the nucleus and necessary stimulus 
for secretory activity, the intrusive atom becoming enclosed by succes- 
sive accretions until it may eventually attain such a size as to preclude 
the closure of the valves and cause the death of the animal. 
But from whatever causes they may originate, these deposits of 
])oarly matter are iiroduced at the expense of the shell, and it is, 
therefore, not surprising to find that the shells of pearl-hearing 
individuals are often deformed, and can usualty be recognized by the 
asymmetry of their valves, the constriction of the lower margin of the 
shell, or the presence of railiatiug grooves or ridges, as it is seldom 
that a regularly formed shell contains a pearl. 
Peai’ls may he formed in almost any part of the tissues, in the sub- 
stance of the mantle, between the body and the gills, between the 
gills and the mantle, or between the mantle and the shell; sometimes 
they are found singly, at others several occur together, either free 
or adherent to the interior of the shell or t() each other. 
