34-8 THE OCEAN WORLD. 



The very numerous division of shells called Asiphonida possesses 

 animals without respiratory siphons. To it belong the shells we shall 

 now describe. The sixteenth family, Unionidse, contains the genera 

 Iridina, Anodon, and Unio. 



The pond mussels, Anodonta, are found in lakes, rivers, and seas of 

 North America, Europe, and Siberia. Their shells are rounded or 

 oval, generally very thin, regular, and equi valve, not gaping, the 

 hinges without teeth, whence their name, from the Greek, oi/oSoVros, 

 without teeth. These shells are nacred inside, and generally smooth. 



Fig. 150. Unio littoralis (Cuvier). 



The Anodonta cygnea (Fig. III., PLATE XV.) is broad, deep, and 

 light ; it is sometimes employed for skimming the cream off milk. 

 The genus is divided into many groups, the principal forms of which 

 are represented in PLATE XV. 



The genus Unio (river mussel) has a wider distribution than 

 Anodonta, and is found in the muddy bottoms of rivers in all the 

 great continents. The animal resembles that of Anodonta, but the 

 shell presents a toothed hinge. The lower face of the valve is nacre- 

 ous, but shaded with purplish violet, and iridescent; the anterior 

 face is of a green colour, which varies from a light to a blackish 

 green. 



Among the species found in European rivers may be noted the 



