BIVALVES. 
25 
FRESH-WATER MUSSELS. 
Family Unionim. 
This group contains several genera of exclu¬ 
sively fresh-water habits. There are only two 
European genera, which are also British,— Unio 
and Anodon , characterized by their oblong shells, 
the mantle-lobes free all round except at the 
posterior side, where the edge is bearded, Ano- 
don is distinguished from Unio by the absence of 
teeth on the hinge-line; hence called edentulous . 
The Unionidce have all large shells; the animals 
bury themselves vertically in the mud of rivers, 
&c., the posterior side upwards. The exposed 
portion of the shell is usually encrusted with a 
calcareous deposit. The umbones, especially of 
Unio , are generally much eroded by the acids dis¬ 
solved in the water. Unios are the most ancient 
of the fluviatile Mollusca. A Unio or Anodon 
appears in the Old Bed Sandstone of Kilkenny. 
They characterize the fresh-water deposits of 
the Purbeck, Hastings Sands, Weald Clay, and 
Middle and Upper Eocene strata. A species, 
U. littoralis , now living in the north of France 
and Sicily, occurs in the fluviatile deposits at 
Clacton, Ilford, and Cropthorn. U. pictorum , 
