164 HENRIK W. WALDEN 



But it is quite evident that no interchange of the endemic European and 

 North American elements in the North Atlantic region took place during 

 the Tertiary. The profound character of the endemism is elucidated by the 

 fact that it is almost total, also on the family level. Besides the Holarctic 

 group (and the doubtful cases of Limax and Cepaea) none of the families, 

 which dominate on the respective sides of the North Atlantic are represented 

 on both. As already mentioned this separation has existed throughout the Ter- 

 tiary; indeed, the initial differences in the late Mesozoic seem to have been 

 even more profound. The few common features which exist, besides the 

 Holarctic group, refer to a more heat-requiring element, for which dispersal 

 via more lengthy routes must be considered. 



On the Pacific side, on the contrary, common features are more manifest. 

 For a number of important families there are clear affinities to Australia and 

 tropical East Asia; these, however, fall beyond the present scope. Pertinent in 

 connection with a probable North Pacific dispersal is a group of five genera 

 (cf. the Notes to Table 1), one of which is now extinct. To be correct, the 

 relationship is not very close. The forms on the two sides of the Pacific are 

 placed in different subgenera, or even genera. This indicates that the time for 

 the interchange must be remote, probably in the Early Tertiary. The genera in 

 question belong to the warm-temperate to Subtropical element, and the 

 later apparent checking of the interchange via the Bering Strait bridge 

 certainly must be seen in connection with the Tertiary climatic development. 

 In the Middle and Late Tertiary the climate in the Bering Strait area apparently 

 was too cold to allow passage for other than cold-adapted, or generally hardy 

 animals. This has been very clearly demonstrated for mammals by Simpson 

 (1940). 



In relation to the total endemic land Gastropoda, in North America as 

 well as in East Asia, this "Amphi-Pacifi" (in fact the recent occurrence of 

 some of the genera is remote from the Pacific coast) element constitutes an 

 inconsiderable fraction only. 



More doubtful is the evidence given by certain other genera which occur in 

 North America and East Asia, though not exclusively. Strobilops, which 

 appears in the North American Pliocene, already existed in Europe in the 

 Eocene, though it became extinct there in the Pliocene. Pilsbry (1948, p. 853) 

 suggests an Asiatic center of origin for this genus, from which it has radiated 

 on the one hand to Europe, on the other to East Asia and via the Bering 

 Strait bridge to North America. However, a careful consideration shows that 

 two of the American subgenera occurred also in the European Tertiary, 

 whereas the genus in East Asia is represented by subgenera which do not live 

 in America. Instead, also one of the East Asiatic subgenera is known from the 

 European Tertiary. As a consequence, the reasons for a North Pacific dispersal 

 of Strobilops scarcely are convincing, whereas it seems difficult to reject the 

 possibility of a trans- Atlantic dispersal within the warm latitudes, the more so 



