THE NAUTILUS. 93 



objects of Csesar'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. The 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, JNIargaritana and Anodohta. 



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, Soland. and Dillw. 

 Union purjnir ens, Say, Desh. Barnes. 

 Union purpuraceus. Lam. 

 Union violaceus, Spengl. 

 Union fluvlatiles, Green. 

 Union {Naia) complanatus, Perkins. 



Shell elongated-oval ; beaks at the aiiterior fourth, almost always 

 eroded ; surface coarsely Avrinkled by the lines of growth, and 



