190 ZOOLOGICAL GEOGRAPHY. [PART III, 
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, whereas 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 Helicide; the Operculated families being 
very poorly represented. Several small genera or sub-genera 
are peculiar to the region, as Testacella (West Europe and 
Canaries); Leucochroa (Mediterranean district); Acicula (Europe); 
Craspedopoma (Atlantic Islands) ; Leonia (Algeria and Spain) ; 
Pomatias (Europe and Canaries) ; Cecina (Mongolia). The largest 
genera are Heliz and Clausilia, which together comprise more 
than half the species; Pupa, very numerous; Bulimus 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. 
Daudebardia 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 Helicide are 
widely distributed. Of the Operculata, Cyclotus, Cyclophorus, 
and Pupina extend from the Oriental region into Japan and 
North China ; 7wdoriais 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 PALAARCTIC 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. 
