19 
point of fact they are obtained only froma very few 
genera of shells. 
None of the air-breathing mollusks (the land snails) 
produce anacreous shell; and among fresh-water mollusks 
none are pearl-bearers, except certain of the bivalves, no- 
tably those belonging to the group appropriately called 
the Naiades, of which the common river-mussel (Unio) is 
atypical example. The soft internal parts of these mol- 
lusks are covered by a thin, delicate membrane called the 
mantle, from the surface of which and particularly from 
its outer edge, material is excreted to form the inner layers 
of the shell. The shell consists of two parts, the epider- 
mis and the shell proper, the latter composed of numerous 
layers. The epidermis, which resembles horn, is chiefly 
composed of a substance called ‘‘ conchioline,”’ and is solu- 
ble in caustic alkalies. 
The pearls of commerce, however, are almost wholly ob- 
tained from bivalve (lamellibranch) shells, of which the 
following families have a nacreous lining: Aviculide, 
Mytilide and Unionide, the latter being wholly fresh- 
water shells, also known as the Naiades. A few families 
of other genera are also brilliantly pearly, but need not be 
here discussed. The true pearl oyster (Meleagrina) of the 
Pacific and Indian Oceans belongs to the first of these 
groups, and has from time immemorial yielded the bulk 
of commercial pearls, while its large and thick shell fur- 
nishes the mother-of-pearl for countless ornamental pur- 
poses. 
The Naiades are of particular interest in this country, 
as itisin North America that this group is the most 
abundant, and it is only of the occurrence of pearls, 
and the preservation of the fisheries of the United States, 
that my paper will treat. Many hundred species of Unio, 
Anodon, etc., have been found in our great rivers and 
lakes ; the Mississippi basin teems with them ; forthe most 
part in forms quite distinct from those of the Atlantic 
