X CIRCUMPOLAR SPECIES 28/ 



which in southern countries are not confined to any particular 

 quality 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 Riftel 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. 



Circum'polar Species. — A certain number of species are com- 

 mon to the extreme north both of the Palaearctic and Nearctic 

 regions, and are, in fact, circumpolar. The number of these 

 species, however, is so small, not exceeding about 40 species 

 ( = 16 genera), that it seems hardly worth while creating a spe- 

 cial 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 geolog- 

 ical 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 : — 



Valvata 



. 1 sp. 



Helix . 



. 4 sp. 



Succinea . 1 sp. 



Physa . . 1 sp. 



Bithynia 



• 1 „ 



Patula . 



.2 „ 



Liranaea . 7 „ 



Anodonta 1 „ 



Vitrina 



. 1 „ 



Pupa . 



. 3 „ 



Planorbis. 5 „ 



Unio . . 1 „ 



Hyalinia . 



. 4 „ 



Cionella 



• 1 „ 



Aplecta . 1 „ 



Pisidium . 1 „ 



G-reat 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 Avion., and 25 Helix., the latter belonging princi- 

 pally to the sub-genera Xerophila., Tachea^ Triehia^ and Fruti- 

 cicola. Three Testacella are probably not indigenous, but are 

 now so well established 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 ( Cyclostoma elegans) occurs in Ireland but not in Scotland, 

 while the other (^Acicula lineata) reaches the southern counties 

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



