DREISSENA. UNIONID^E. 



407 



aggregated masses, attached to the bottom by the byssus. But 

 it is also found in many of the large Continental rivers, such as 

 the Danube, the "Wolga, and the Rhine; and it thus seems 

 equally capable of living in fresh water. It was first discovered 

 in England a few years since 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 accomplished by its adhesion to the 

 keels of boats. 



ORDER V. UNIONID^E. 



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

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

 Anodon and Unio. 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- 

 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 ; 



FIG. 592. ANODON DIPSAS. 



