20 
yater-shed and of the Old World. The Unios, while all 
iridescent, vary greatly in tint, exhibiting all the delicate 
shades of pink, brown, purple, etc., as well as white. The 
rivers of Europe, of Mesopotamia and of China, also yield 
large numbers of Unios ; while the allied genera Hyria and 
Castalia are found in the rivers of South America. 
The same causes and operations that result in the pro- 
duction of pearls, or free nacreous concretions in the soft 
animal substance of the pearl-oysters or mother-of-pearl 
shells, also produce in a modified way the tuberculous or 
knob-like protuberances and irregularities of surface that 
are frequently seen on the pearly inner surface of the 
valves and projecting therefrom. The flatter or less pro- 
nounced forms of these nacreous excrescences are often 
salled ‘‘ blister pearls,’’ because of their resemblance to 
vesicular eruptions, or water blister, caused by burns. 
Fresh-water pearls are found, as before stated, in vari- 
ous species of Unios, more frequently, according to Dr. 
Isaac Lea, in the Unio complanatus, but also in the fol- 
lowing: U. blandianus, U. buddianus, U. costatus, U. el- 
liotti, U. fragilis, U. globulus, U. gracilis, U. mortoni, U. 
nodosus, U. orbiculatus, U. ovatus, U. torsus, U. undula- 
tus and U. virginianus. Not one pearlin a hundred from 
the Unios is of good shape, and probably not more than 
one in a hundred is really fine; therefore, as the worth of 
a pearl depends on lustre and form, the greater number ob- 
tained from this source are of slight value. 
A hundred pearls have been found ina single shell ; but 
as a rule these have little or no value. Very curious 
nacreous groups made of many small pieces are at times 
found attached to the hinge, but these are generally with- 
out sufficient lustre to be of value, and are rarely collected. 
These groups are caused by the aggregation of many small 
ones cemented by a deposit of nacre, and are often half an 
inch across. 
In color, the Unio pearl presents an extended series of 
