THE AUTOBIOGRAPHY O'P A SWAN-MUSSEL. 39 
“ The nacreous layer covers the whole surface of the interior, 
and is iridescent in appearance and very beautiful. It is com¬ 
posed of calcoreous laminae, alternating with organic, and owes 
its iridiscent appearance to the diffraction of light by the free, 
irregular edges of the calcareous laminae. Muller believes that 
our shell increases by the act of intussuception, and mentions 
as an argument in its favour the fact that the surface of the 
mantle is separated from the shell over a large area, viz., 
between the pallial line and the ridge bordering the ligament 
where the muscles are attached, and between the two adductors; 
while Fulberg thinks that the organic part of our shell is pro¬ 
duced by fibrillation of the cells, so that what with these and 
many other variable opinions, it is difficult for a swan-mussel 
to have an opinion at all. Even the ligament which connects 
the two portions of my shell together is much more complex 
than it appears on a cursory glance. It consists of two parts— 
an outer and an inner. The outer is laminated like the 
epiculinda ,brio which it gradually passes. But the inner part 
is striated in a radial manner, and consists of fibres composed 
of two different substances, each reflecting light differently. 
This inner portion has a ridge, to which are attached the 
muscles of the foot, thus serving the same purposes as teeth do 
in your race, my friend,” addressing the Unio . “Muller 
regarded the inner-striated portion of the ligament as probably 
being continuous with the nacreous layer. Sometimes a grain 
of sand finds its way into our mantle cavity, where, by the 
deposition of nacre round it, we transform it into a beautiful 
object called by men a pearl,* and which they hold in great 
value and esteem ; but- 
Here the Unio interrupted the discourse by breaking in with, 
u Allow me to correct you, Madam; your pearls have never 
fetched any high price. It is my near kinsman, the Unio 
Margaritifer , you are thinking of, and whose very name signifies 
pearl-bearing. One of our historians, named Brown, states that 
the pearls sent from the river Tay, in Perthshire, where my 
relation lives, to London, from, the year 1761 to 1764, were 
worth ^'io,ooo.”t 
* They are generally formed in and around the pericardium, the name given 
to the membranous sac enclosing the heart, 
f Other instances may be cited The following are interesting :—Camden 
says : “ Higher up the little river Tut runs into the-sea, in which the shellfish, 
having by a kind of irregular motion taken in the dew, which they are 
extremely fond &f are impregnated and produce pearls , or, to use the poet’s 
phrase, ‘ bacca concha,’ shell berries, which the inhabitants, when the tide is 
out, search for, and our jewellers buy of the poor for a trifle and sell again at 
a very great price.” The italics are mine. Several old authors mention the 
formation of pearls by dew as in the above. Julius Caesar came to England 
for pearls, and Pliny tells us that he obtained enough to cover a breastplate, 
which he gave to Venus Genatrix, who hung it in her temple at Rome. 
