THE NAUTILUS. 93 
objects of Czesar’s expedition to Great Britain was to obtain pearls 
from the fresh-water clams of that country. The pearl fisheries of 
Scotland in the river Tay, were continued until the end of the last 
century and many large and beautiful pearls were found in the 
river Tyronne, in Ireland. 
One of the finest of the English pearls is now in Queen Victoria’s 
crown. Old and deformed specimens are the most liable to contain 
pearls, and they consist of a nucleus of some foreign substance 
such as a grain of sand, covered by successive layers of nacre 
secreted by the mantle of the animal. he color of the pearls varies 
with its species, and is of the same shade as the nacre which lines 
the interior of the valves. 
In China the natives make little flat lead casts of their idols; 
these they insert between the shell and the mantle of the animal, by 
prying open the valves of these clams with a wedge; the presence 
of these foreign substances irritates the animal and causes it to 
deposit layer after layer of nacre upon them. After a time the 
shells are opened and the images removed and worn as charms. 
This family is divided into six genera, three of which inhabit the 
U.S.: they are called Unio, Margaritana and Anodonta. 
Genus Unio, Retzius, 1788. 
Shell equivalve, multiform ; hinge with a short, irregular, striated, 
simple or divided tooth in each valve, and an elongated, marginal 
tooth. 
There are about one thousand species of this genus, nearly eight 
hundred of which are American and one hundred and fifty or more 
inhabit the Ohio river and its tributaries; only five of these are 
found in New England and but three inhabit R. I. 
187.— Unio complanatus, Solander. 
Syns. : 
Mya complanata, Soiand. and Dillw. 
Union purpurens, Say, Desh. Barnes. 
Union purpuraceus, Lam. 
Union violaceus, Spengl. 
Union fluviatiles, Green. 
Union (Naia) complanatus, Perkins. 
Shell elongated-oval; beaks at the anterior fourth, almost always 
eroded; surface coarsely wrinkled by the lines of growth, and 
