348 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, Unionidz, contains the genera 
Iridina, Anodon, and Unio. 
The pond mussels, Anodonta, axe found in lakes, rivers, and seas of 
North America, Europe, and Siberia, Their shells are rounded or 
oval, generally very thin, regular, and equivalve, not gaping, the 
hinges without teeth, whence their name, from the Greek, avéddvros, 
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 PLarE 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 
