_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 Helicidæ; 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 _Helix_ 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 countries, 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 Helicidæ 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 distributed. THE PALÆARCTIC SUB-REGIONS. The four sub-regions which are here adopted, have been fixed upon as those which are, in the present state of our knowledge, at once the most natural and the only practicable ones. {191}No doubt all of them could be advantageously again subdivided, in a detailed study of the geographical distribution of _species_. But in a general work, which aims at treating all parts of the world with equal fulness, and which therefore is confined almost wholly to the distribution of families and genera, such further subdivision would be out of place. It is even difficult, in some of the classes of animals, to find peculiar or even characteristic genera for the present sub-regions; but they all have well marked climatic and physical differences, and this leads to an assemblage of species and of groups which are sufficiently distinctive. _I. Central and Northern Europe._