218 Flow to obtain tt. 
They are often found in canals and similar habitats ; they are 
abundant in the Midlands, but several attempts that have been 
made to introduce them into more northern waters have proved 
abortive. 
Fig. 38. Anadonta cygnea. 
On the bottoms of ponds and streams we should certainly 
cultivate the bivalves, or Conchifera. The largest are the swan 
mussel (Axadonta cygnea) and its varieties. They attain a length 
of about six inches, and are not of a high food value when mature, 
but when young they are eaten with avidity by many fishes. In 
their infancy they are parasitic on fish, but their attacks do not 
Fig. 39. Unio margaritifer. 
appear to be at all hurtful, beyond slightly impeding the progress 
of the fish to which they are attached in their movements through 
the water. 
The Unionidae are similar bivalves, not quite so large as the 
