VERTIGO. 73 



species is known from as far south as the 35th parallel, and 

 only a few species (in Algeria and Tunis) approach it. 



Where Vertigo occurs in arid regions it inhabits humid 

 places, usually at high levels. Thus, in the arid zone of south- 

 ern New Mexico and Arizona, Vertigo is often abundant in 

 the forests mainly above 8000 ft., where there is abundant 

 shade and moisture. It may usually be found where there 

 are aspens. 



Part of the species of temperate latitudes of Europe and 

 America are identical or very similar. Their distribution 

 probably dates from the Pliocene. Doubtless much if not all 

 of the evidence of the route has been destroyed by glacial 

 action. While the present distribution of these closely allied 

 forms suggests emigration around the north Atlantic, a 

 Pliocene distribution through northern Asia seems far more 

 likely, though northern Pliocene fresh-water deposits which 

 might contain traces of this fauna are still unknown. 



This interpretation is in harmony with the facts of present 

 distribution relative to the circumpolar modesta group, where 

 the data indicate continuity of distribution within post- 

 Pliocene time from Alaska west to Lapland, and a southern 

 movement during Glacial time. The other Vertigines, less 

 tolerant of cold and for the greater part more differentiated, 

 no doubt were pressed south earlier, and were exterminated 

 in Siberia and the mountain states of western America. The 

 survivors are chiefly at the extremes of their former range, in 

 Europe and in the eastern half of North America. 



Vertigo pygnuca is to be regarded as a member of that 

 Pliocene fauna, still unchanged in the two hemispheres, though 

 in both giving rise to various divergent races. V. ovata and 

 antivertigo are but slightly differentiated. V. moulinsiana 

 and V. ventricosa differ slightly more, and a few other species 

 could be similarly paired. In both hemispheres there are also 

 far more differentiated forms of Vertigo, mainly southern in 

 distribution, indicating a long period of evolution in America 

 as well as in Europe. 



Where Vertigo arose may perhaps never be known. The 

 presence of typical species in the Oligocene of Central Europe, 



