204 BULLETIN OF THE 



southern extension of the Region we find the Interior Region overlap- 

 ping, as will be shown below while treating of the Interior fauna. At 

 other points in the Region, also, have been found species from the In- 

 terior Region,* especially small Zonites, which are able to bear the 

 severe climate of the north. 



The following are the species of the Northern Region : — 



Vitrina limpida. Helix pulchella. 



Angelicae. Cionella subcylindrica. 



exilis. Pupa muscorum. 

 Zonites fulvus. Blandi. 



nitidus. Hoppii. 



viridulus. decora. 



Fabricii. borealis. 



milium. Vertigo Gouldi. 



Binneyanus. Bollesiana. 



ferreus. simplex. 



exiguus. Punctum minutissimum. 



multidentatus. Succinea Haydeni. 

 Patula striatella. Verrilli. 



asteriscus. Higginsi. 



pauper. Groenlandica. 



Helix harpa. Totteniana. 



Of the above, several are circumpolar species, common to the three 

 continents of Europe, Asia, and America. There being no mountain- 

 barriers in these regions, they are not restricted in their range across 

 America. In their progress southward, also, they have met with no 

 transverse mountain-barriers, but have spread equally on the east and 

 west of the Rocky Mountains and Sierra Nevada. Hence we find them 

 common to the whole of North America-t Such are : — 



* Sec Proc. Phila. Acad., N. S., 1861, p. 330, for the northern range of species 

 from the Interior Region. 



t In the same way we can account for the distribution of the small eastern species 

 over the Central and Pacific Provinces. They have not crossed the mountain-bar- 

 riers, but spread southward from their wider range in the north. Such are: — 



Zonites arboreus. Limax campestris. 



indentatus. Patula striatella. 



minusculus. Helix lineata. 



milium. Punctum minutissimum. 



These northern species, both indigenous and circumpolar, may have been assisted 



