DREISSENA. UNIONID^. 407 



large Continental rivers, such as the Danube, the Wolga, and 

 the Rhine. It was first discovered in England in 1824, in the 

 Commercial Docks of London, where it was probably conveyed 

 with some timber ; and it has since diffused itself through the 

 rivers and canals of the whole island. This may be partly ac- 

 complished by its adhesion to the keels of boats. The young 

 even pass through the pipes laid down for. the supply of water 

 and thus make their appearance most unexpectedly in tanks, &c. 



SECTION E. UNIONID^E. 



1033. The Mollusks of this Order, frequently called Fresh- 

 water Mussels, are for the most part included in the genera 

 Anodon and Unto. The former is named from the absence of 

 teeth in the hinge, which has merely a ligament along its entire 



length. The ani- 

 mals are without 

 a byssus ; and 

 creep over the 

 sand or mud by 

 means of their 

 large foot, which 

 has three layers 

 of fibres disposed 

 in different direc- 

 tions, so that it 

 has considerable 

 power of altering 

 its form and di- 

 FIG. 692.-ANODONBIPSA8. mensions. They 



are most commonly found sunk in mud, however, with that part 

 of the aperture of the shell which corresponds to the mouth 

 directed upwards. It has been said that they have some power 

 of swimming, by striking the water with their valves. The 

 Anodon is viviparous, the eggs being hatched within the shell ; 

 and thousands of young ones may be seen in the winter, with a 



