94 



notch, in which the anterior extremity of the ligament 

 is sunk or attached ; their apices, like those of the Unio, 

 are eroded. The substance of the shell is pearly, and 

 covered with a false epidermis; the valves are general- 

 ly very thin, concave, and attain occasionally a great 

 size. Shell equivalve, transverse, inequilateral, two 

 distinct muscular impressions; ligament external, insi- 

 nuated at the anterior extremity into the sinus of the 

 cardinal ridge. 



It inhabits ponds and lakes, and the animal is sup- 

 posed to be viviparous. 



JSo distinct angle at the posterior extremity of the car- 

 dinal line 



Anodon cygnea Anodon rubens 



anatina crispata 



sulcata uniopsis 



fragilis Pensylvanica 



cataracta intermedia 



With a distinct angle at the posterior extremity of the 

 cardinal line. 



Anodon trapezialis Anodon glauca 



exotica sinuosa 



Anodon Patagonica 



