190 ZOOLOGICAL GEOGRAPHY. [part in. 



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

 purposes of geographical 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, but comparatively scarce towards the North. 

 South Europe alone possesses over 600 species, wmereas there are 

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

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

 the great majority are Helicidse; the Operculated families being 

 very poorly represented. Several small genera or sub-genera 

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

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

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

 Pamatias (Europe and Canaries) ; Cecina (Mongolia). The largest 

 genera are Helix and Clausilia, which together comprise more 

 than half the species ; Pupa, very numerous ; Bidimus and 

 Achatina in moderate numbers, and all the rest small 

 Helix is the only genus which contains large and handsome 

 species ; Bulimus and Achatina, so magnificent in tropical coun- 

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

 Daudehardia is confined to Central and South Europe and New 

 Zealand ; Glandina is chiefly South American ; Hyalina is only 

 American and European ; Buliminus ranges over all the world 

 except America ; and the other European genera of Helicidae are 

 widely distributed. Of the Operculata, Cyclotus, Cyclophorus, 

 and Pupina extend from the Oriental region into Japan and 

 North China ; Tudoria is 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. 



The Pal^earctic Sub-regions. 



The four sub-regions which are 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. 



