202 THE FEESH-WATER MUSSEL [CH. VI 



been conveyed against the stream along the waterways of 

 commercial traffic. 



Anodonta and the other Unionidce having been fresh- 

 water animals for long ages, have availed themselves of the 

 superior activity and power of dispersal possessed by fish. 

 The glochidia when attached to their host may be carried 

 many miles from the place of their birth. It is even con- 

 ceivable that they may have been transported from one 

 stream to another by such fish as make periodic journeys 

 to the sea. It would be extremely interesting to try how 

 far the glochidium during its parasitic life is tolerant of 

 salt water. The experiment could be conducted with some 

 species of stickleback (Gasterosteus) which live indiffer- 

 ently in fresh or in brackish water. 



BRITISH FRESH-WATER LAMELLIBRANCHS. 

 Class Lamellibranchia (Pelecypoda). 



ORDER EULAMELLIBRANCHIA. 



Family UNIONIDCE. Shell equi valve, oblong, inequilateral, pearly 

 within ; large external ligament; anterior hinge-teeth thick 

 and striated, posterior teeth often absent ; all hinge- teeth oc- 

 casionally rudimentary ; adductor muscle-impressions deep, 

 genus Unio, hinge-teeth^ strongly marked, lunule (depressed area 



in front of umbo) distinct, shell thick, solid. 

 U. margaritifer (L.), 5" x 2f", oblong, lower margin straight. 

 U. pictorum (L.), 3" x l", oblong, urn bones with small tubercles 

 not confluent, anterior teeth compressed and crenulated. 



