264 DIMYARIA.—UNIONIDA. 
GENUS UNIO. 
The condition of the hinge affords the distine- 
tive character in this family. The genus Unio has 
it furnished with a short crested anterior tooth, 
and a lengthened posterior lateral one in the right 
valve, shutting between two similar teeth in the 
opposite. 
The shells of this genus, which are frequently 
large and richly pearly in the interior, are fre- 
quently used by artists for containing their colours. 
One of our British species is hence named Unio 
poctorum, or the Painter’s Unio. Many of them 
yield pearls, which are generally superior to those of 
the marine Mussels. Our finest native species, hence 
named UW. margaritiferus, is one of these. It is 
about five inches long, and half as broad, some- 
what kidney-shaped, covered with a black epider- 
mis, usually worn away at the beaks; the interior 
1s pearly, but not brilliant. This species inhabits 
the rapid streams of mountainous districts in vari- 
ous parts of the British Islands. 
“The Pearl Mussel, as this mollusk is familiarly 
called, enjoys a distinguished reputation as one of 
the few indigenous bivalves which yield the beau- 
tiful productions whose name it bears. In ancient 
times Britain enjoyed some celebrity for its pearls, 
as they constituted one of its attractions for Julius 
Cesar, who, however, does not seem to have reaped 
a very rich harvest, so far as quality went, though 
he obtained enough in quantity to cover with them 
a buckler, which he dedicated to Venus Genetrix, 
and suspended in her temple. ‘The pearls used for 
the construction of his present were probably such 
as Roman ladies would have scorned to wear, 
