28 LAND AND FRESH-WATER MOLLUSKS. 5 
The Pearl Mussel enjoys a reputation as one 
of the few British bivalves which contain the 
beautiful production whose name this species 
bears. The other Unios and the Anodon occa- 
sionally yield pearls, as also the marine mussel 
(Mytilus edulis) and the oyster. 
Pearls are of the same nature as the nacreous 
layer of the shell, and are abnormal secretions 
of the mantle, composed of alternate layers of 
animal membrane and calcareous matter, de- 
veloped around some foreign body,—a grain of 
sand, a parasite, or an unfertilized ovum. 
The great Linnzeus owed in part his elevation 
to nobility to a discovery of causing this fresh- 
water mussel to produce pearls at pleasure. This 
was accomplished, it is conjectured, by boring 
small holes through the shell and introducmg a 
particle of sand, which would become a nucleus 
round which a pearl would be developed; but 
the artificial production of pearls had been long 
known to the Chinese. The Avicula margariti- 
fera of the Indian seas is the most famous for 
pearls. 
Pearls have been associated with the name of 
Britain from the very earliest known times. Sue- 
tonius gives as the reason for Ceesar’s expedition 
into Britain, the search for pearls, which Pliny 
seems to confirm, saying that Ceesar gave a breast- 
plate covered with British pearls to Venus Genitrix, 
