156 HENRFK W. WALDEN 



North America. From its early beginning it seems to be entirely distinct 

 from the Palearctic one. At the end of the Mesozoic, tropical genera dominate, 

 which seem to be purely American. Already some early representatives of the 

 two major endemic groups appear. These increase rapidly, whereas the 

 Tropical genera decrease correspondingly. Soon, in the Eocene, the propor- 

 tions between endemic North American and essentially Tropical genera are 

 near those of the recent period. At the same time the first representatives of 

 the Holarctic genera appear in the strata. 



In addition to these trends in the development it is striking that the number 

 of now extinct genera has always formed a minor fraction. The recent fauna, 

 essentially, is composed of direct descendants of the early Tertiary fauna, 

 which have been able to adapt themselves from the original, almost tropical, 

 conditions to the present temperate ones. 



Henderson (1931) suggests as the basic reason for the American endemic 

 pattern the probably profound separation of land masses in the Cretaceous. 

 The largely remaining differences between the eastern and western gastropod 

 groups have been maintained by unfavorable conditions in a broad area, 

 east of the Rocky Mountains, which seem to have existed rather continuously 

 since the Eocene. The principal exchange of land Gastropoda has taken place 

 from the east to the west. The forcing of the Rocky Mountains by some 

 eastern genera had largely come to an end in the Miocene, resulting in a 

 today partially highly endemic offshoot from the eastern group. The effect of 

 the interchange between the two major endemic groups never reached beyond 

 the North American continent. 



Table 1 gives an account of the extracontinental relations on the generic 

 level for the North American land Gastropoda. As the biogeographical 

 boundary of North America the border of Mexico has been used, in accor- 

 dance with Pilsbry (1939-48). Only six of the essentially endemic North 

 American genera exceed this border. 



The dispersal of the tropical American element falls beyond the present 

 theme, and will not be discussed further. It comprises about 50 species, most 

 of them in the extreme south of North America. The Tertiary Tropical 

 gastropod fauna was composed largely of species closely related to the recent 

 ones, but the latter are late invaders, probably mainly Post-glacial. The signs of 

 endemism are weak, or non-existent. 



For the remaining non-endemic element three possible ways of dispersal 

 must be considered : 



(1) The North Pacific route. 



(2) The North Atlantic route. 



(3) Across the subtropical or tropical parts of the Atlantic. 



The Holarctic genera comprise somewhat over 100 species in North 

 America. These represent all stages from morphological identity in the New 



