AKODON. 
271 
on the edge ; foot (c) compressed, ovate, sub- 
quadrate. 
Shell oblong-elongate, equivalve, inequilateral, solid, 
internally pearly, covered with a hard olive or 
black horny periostracum ; hinge without any 
true cardinal teeth, with irregular anterior, and 
long laminar posterior lateral teeth, or tooth- 
less; muscular scars numerous ; ligament and 
cartilages external, elongate, strong. 
These animals live sunk perpendicularly in the 
mud of rivers, with the front end downwards, and 
the siphonal edge even with the surface of the mud, 
but sinking themselves deeper when disturbed ; they 
are also found under the shelter of stones in rivers 
and running water with stony banks. 
The family contains three genera 
1. Anodon, Hinge toothless. 
2. Alasmodon, Hinge with short anterior teeth. 
3. Unio, Hinge with short anterior and long pos- 
terior teeth. 
1. Anodon Lam, (Fresh-water Muscle.) 
Shell oblong, thin, rather compressed behind ; hinge 
margin toothless ; gills formed of waved tubes, 
forming a kind of trellis. 
127. 1. Anodon cygneus. Swan Fresh-water Mus- 
cle. — Shell oval, tumid, rounder and ventri- 
cose in front, compressed and more or less 
angular above behind, covered with an olive 
periostracum. (t. 1. f. 8.) 
