190 ZOOLOGICAL GEOGRAPHY. [part hi. 



They are, therefore, at present of comparatively little use for 

 purposes of geograpliical distribution, even were it advisable 

 to enter into the subject in a work which will, perhaps, be too 

 much overburdened with details only of interest to specialists. 



Land Shells. — These are very numerous in the warmer parts 

 of the region, Init comparatively scarce towards the North. 

 South Europe alone possesses over 600 species, whereas there are 

 only 200 in all Northern Europe and Asia. The total number 

 of species in the whole region is probalily about 1,250, of which 

 the great majority are Helicidic ; the Operculated families being 

 very poorly represented. Several small genera or sub-genera 

 are peculiar to the region, as Tcstacella (West Europe and 

 Canaries); Leucorhroa (Mediterranean district) ; Acicula (Europe); 

 Craspcdoijovm (Atlantic Islands) ; Lconia (Algeria and Spain) ; 

 Pomatias (Europe and Canaries) ; Cccina (Mongolia). The largest 

 genera are Helix and Clansilia, which together comprise more 

 than half the species ; Pi/jya, very numerous ; Bidimvs and 

 Achatina in moderate numbers, and all the rest small. 

 Helix is the only genus which contains large and handsome 

 species ; Bidimus and Achatinet, so magnificent in tropical coun- 

 tries, being here represented by small and obscure forms only. 

 Daudchardiei is confined to Central and South Europe and New 

 Zealand ; Glandina is chiefly South American ; Hycdina is only 

 American and European ; Bidiminvs ranges over all the world 

 except America ; and the other European genera of Helicidfe are 

 widely distributed. Of the Operculata, Ct/clotus, Cyclophorns, 

 and Pupina extend from the Oriental region into Japan and 

 North China ; Tudoriais found in Algeria and the West Indies ; 

 Hydrocena is widely scattered, and occurs in South Europe and 

 Japan. The genera of freshwater shells are all widely dis- 

 tributed. 



Til?: rAL.'EARCTrc Sub-regions. 



The four sub-regions which arc here adopted, have been 

 fixed upon as those which are, in the present state of our know- 

 ledge, at once the most natural and the only practicable ones. 



