CIRCUMPOLAR SPECIES 28/ 



which in southern countries are not confined to any particular 

 ([uality of soil are in more northern latitudes found only on 

 limestone, which absorbs more heat than other formations. Con- 

 versely, the higher elevations of the Alps, Pyrenees, and even 

 Carpathians are like islands in a sea, and support a thoroughly 

 northern fauna, quite strange to that of the plains below. Thus 

 Helix harpa Say, a completely boreal shell, which is at home in 

 Canada, Sweden, Lapland, and the Amoor district, is found on 

 the Eiffel Alp, at a height of 6000 feet.^ Vertigo arctica Wall., 

 a species abundant in Lapland, North Siberia, Iceland, and Green- 

 land, occurs on the high Alps of the Tyrol. 



Circumsolar- Species. — A certain number of species are common 

 to the extreme north both of the Palaearctic and Nearctic regions, 

 and are, in fact, circumpolar. The number of these species, how- 

 ever, is so small, not exceeding about 40 species ( = 16 genera), 

 that it seems hardly worth while creating a special sub- 

 region for their reception, particularly as no genus is peculiar. 

 At the same time the fact is instructive as illustrating the close 

 connexion of the northern districts of the two regions, a connexion 

 which was no doubt more intimate in recent geological times than 

 it is now. 



The circumpolar genera are as follows. The list decisively 

 sets forth the superior hardiness of the fresh-water as compared 

 with the land genera : — 



Great Britain. — There are in all about 130 species — 83 land, 

 46 fresh- water ; Limnaea involuta (mountain tarn near Killarney) 

 appears to be the only peculiar species. There are 11 Hyalinia, 

 5 Arion, and 25 Helix, the latter belonging principally to the 

 sub-genera Xcrophila, Tacliea, Trichia, and Fruticicola. Three 

 Testacella are probably not indigenous, but are now so well estab- 

 lished as to reckon in the total. Of the four Clausilia two reach 

 Ireland and one Scotland ; two do not occur north of the Forth. 

 There are only two land operculates, one of which (Ci/clostoma 

 elegans) occurs in Ireland but not in Scotland, while the other 

 {Acicula lineata) reaches the southern counties of Scotland. 



' Craven, Journ. de Conchyl. (-3) xxviii. p. 101. 



