vi] DISTRIBUTION OF SELECTED GROUPS 91 



MOLLUSCS. 



Of bivalves only the Unionidae and Cydadidae 

 are freshwater families, both with world-wide range. 

 Of the marine Mytilidae, Dreyssena polymorpha is 

 the only freshwater species. Its home is the Volga 

 basin, whence it became accidentally introduced into 

 Western Europe in the 18th century. It is now in 

 the interesting stage of spreading through all rivers 

 and even into land-locked lakes. Those which ascend 

 the Danube basin are meeting others from the Rhine 

 which they have reached by Baltic shipping. 



The true mother-of-pearl, genus Meleagrina, 

 ranges from the coast of Persia to Celebes and 

 Melanesia, reappearing on both coasts of Mexico 

 and Central America. 



FRESHWATER CRABS AND CRAYFISHES. 



The freshwater Decapods have been studied 

 exhaustively by Ortmann, who in a classical paper 

 has based upon their distribution the most suggestive 

 reconstructions of Cretaceous and Tertiary geography. 



Crayfishes are older than crabs, and they can 

 inhabit cold regions, whilst the crabs are essentially 

 tropical. The crabs comprise (1) the Potamoninae 

 in Africa and Madagascar, India and China, Malay 

 and Papuasian islands ; with their northern limit 

 South Japan, Asia Minor, Greece and South Italy ; 



