380 MANUAL OF THE MOLLUSCA,. 
give the pearly lustre, which appears to depend on minute 
undulations of the layers, represented in Fig. 23. This lustre 
has been successfully imitated on engraved steel buttons. 
Nacreous shells, when polished, form ‘mother of pearl ;” 
when digested in weak acid they leave a membraneous residue 
which retains the original form of the shell. This is the most 
easily destructible of shell-textures, and in some geological 
formations we find only casts of the nacreous shells, whilst 
those of fibrous texture are completely preserved. 
Pearls are produced by many bivalves, especially by the 
Oriental pearl-mussel (avicula margaritifera), and one of the 
British river mussels (unio margaritiferus). They are also found 
occasionally in the common oyster, in anodonta cygnea, pinna 
nobilis, mytilus edulis, or common mussel, and in spondylus 
gederopus. In these they are generally of a green or rose 
colour. The pearls found in arca noe are violet, and in anomia 
cepa purple. ‘They are similar in structure to the shell, and, 
like it, consist of three layers; but what is the innermost layer 
in the shell is placed on the outside in the pearl. The iridescence 
is due to ight falling upon the out-cropping edges of partially 
transparent corrugated plates. The thinner and more trans- 
parent the plates the more beautiful is the iridescent lustre ; 
and this is said to be the reason why sea pearls excel those 
obtained from fresh-water molluscs. Besides the furrows 
formed by the corrugated surface there are a number of fine 
lark lines (+7oo inch apart), which may add to the lustrous 
effect. In some pearls these lines run from pole to pole like 
the longitudes on the globe; in others they run in yarious 
directions; and in a few the lines on the same pearl haye 
different directions, so that they cross each other. The nucleus 
frequently consists of a fragment of a brownish-yellow organic 
substance, which behaves in the same way as epidermis when 
treated with certain chemical re-agents. Sand is generally said 
to be the nucleus; but this is simply a conjecture which has 
etadually become regarded as a fact; it is quite the exception 
for sand to be the nucleus; as a general rule it is some organic 
substance. In some districts one kind of nucleus seems to be 
more common than another; at least, this is how the different 
results obtained by observers in different localities may be 
explained. Filippi (Swill? origine delle Perle. Translated in 
Miiller’s Archiv. 1856) found distoma to be the nucleus in many 
cases; Kuchenmeister found that the pearls were most abundant 
in the molluscs liying in the still parts of the river Elster, where 
the water-mites (limnochares anodonte) existed most nume- 
