ANODON. 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.) 



