THE EUROPEAN SEAS. 



261 



its fauna, is not a mere fanciful speculation, but 

 will be found to be collaterally supported by many 

 independent considerations. 



Generic assemblages of plants and animals, whether 

 terrestrial or aquatic, whether fresh-water or marine, 

 have their regions, or definite geographical areas : 

 these are what are known as " generic areas." Each 

 of these has its " metropolis/' or district of greatest 

 number, either of tropical or specific forms ; geo- 

 graphical unity seems to be one of the essentials of 

 every generic group. 



The genus Mitra offers a good illustration 

 of this geographical grouping. These shells have 

 their head-quarters in the Indo- Pacific Ocean ; and 

 they are thence distributed, but in decreasing 

 numbers, in every direction away from that central 

 region. Typical species of " Mitre-shells " from the 

 Indian Ocean are to be met with throughout the 

 Red Sea. Numerous other forms of the genus are 

 found on the west coast of Africa, and about the 

 Atlantic islands. As many as eleven species live in 

 the Mediterranean, which are also mostly common 

 to the Atlantic ; but this is their present northern 

 limit. These species do not range up the Lusita- 

 nian coasts, so that their European range is dis- 

 tinctly defined. 



On going back to an earlier facies, or period of 

 the present Atlantic fauna, "Mitre shells" of large 

 and handsome forms are met with in the Faluns of 

 Dax and Bordeaux, as in those of Touraine, where 



