154 HENRIK W. WALDEN 



with Hibbard and Taylor (1960); Licharev and Rammelmeyer (1952); and 

 Taylor (1954). 



It may also be pointed out that, diverging from Wenz and Zilch, the 

 subgenus Calidivitrina has been placed under Vitrina (cf. information given 

 by Hubendick, 1953). Consequently Semilimax has been regarded as an 

 endemic European genus. Lehniannia has been classified as a subgenus of 

 Umax (cf. Walden, 1961). Finally, the records assigned to the highly problem- 

 atic genus Brachyspira have been disregarded. 



As it seems still to be current in the literature, attention Is drawn here to an 

 old opinion thai the early European gastropod fauna has much In common 

 with the recent American (cf. Ehrmann, 1914. and authors mentioned by 

 him). It was based on superficial shell resemblances, but its ground has 

 nowadays been radically demolished by the gradually deepened taxonomical 

 knowledge. It is not excluded that future taxonomical revision will corre- 

 spondingly modify the conception of some further taxa, whose common 

 presence on both sides of the Atlantic today seems rather enigmatic. 



THE NORTH AMERICAN LAND GASTROPOD FAUNA AND ITS 



DEVELOPMENT 



in all, more than 750 terrestrial gastropod species are known from North 

 America. Of these, about 50 have been introduced by man in historical time. 

 The occurrence of this element has been so exhaustively elucidated by 

 different authors (see Lindroth, 1957; Pilsbry, 1939-48; and Quick, 1952) 

 that it can be disregarded in the present account. The remaining more than 

 700 species are dispersed through 107 genera. 



The North Anierican land Gastropoda are dominated by two pronouncedly 

 endemic groups, which together comprise about 80 per cent of the indigenous 

 species. In relation to the European Gastropoda the endemism occurs largely 

 on the family level. — The first group is eastern, with the Appalachians as its 

 center, and comprises about 250 species. Among these the members of the 

 autochtonous family Polygyridae dominate. Only a minor fraction of the 

 endemic species, about 50 in all, have Old World relatives. In most instances, 

 however, those belong to different genera or subgenera. 



The other. West American group, comprises about 300 endemic species, 

 localized on the Pacific coast and in the Rocky Mountains. This group is 

 dominated by two families of very old, autochtonous development, viz. the 

 Helminthoglyptidae and the Camaenidae. The latter is represented by other 

 genera in Australia and East Asia. Holarctic genera are less well represented 

 in western North America than in eastern. On the contrary, the influx of 

 central or South American genera is much richer in the southwest as a conse- 

 quence of the land connection there. 



Figure I illustrates the development of the terrestrial gastropod fauna in 



