EXPLANATION OF MAP. 



The Map, facing page 1, is intended to show the boundary of the Cauca- 

 sian province of molluscan distribution over which the British species range, 

 and the position wbich it occupies on the surface of the globe with reference 

 to the general system. The student is sixpposed to be acquainted with 

 the geography of the two hemispheres, or to have a geographical atlas 

 before him for reference. The lines of latitude and longitude, and the 

 names of places, are excluded, in order to exhibit more clearly the physical 

 features to which it is desired to call attention ; namely, to the mountains 

 and rivers, the configuration of the land and water, and to the parallels of 

 equality of temperature, copied from Humboldt's system of isothermal 

 lines. An attempt has been made by the lithographer to indicate the 

 specific centre of the province by deepening the colour of that part of it in 

 which the most characteristic of the genera and species congregate, and 

 are most prolific in individuals ; the part, in short, in which the genera 

 and species obtain their maximum of development. 



The primary division of the terrestrial portion of the globe into broadly- 

 defined provinces of molluscan distribution, is as follows : — 



Eastern Hemisphere. 

 The Caucasian Province. 

 The West African Province. 

 The South African Province. 

 The Malayan Province. 

 The Australian Province. 



Western Hemisphere. 

 The North American Province. 

 The Columbian Province. 

 The-Brazilian Province. 

 The Bolivian Province. 

 The Chilian Province. 



The Caucasian Province is divided to some extent in its western part by 

 the mountain chain of the Pyrenees, Alps, and Carpathians, but this division 

 does not affect the range of the species of land and freshwater mollusks 

 which inhabit Britain. The region north of that apparently natural barrier, 

 extending to the utmost limit of molluscan life towards the pole, is termed 

 the Germanic Region. The southern portion, reaching to the boundary of 

 the province in the opposite direction, at about the 15th parallel of north 

 latitude, on the west side of Africa, and extending on the east side to 

 Abyssinia and the island of Socotra, is called the Lusitanian Region. But 

 the whole of Britain on the Continental side is surrounded by Grermanic 

 forms, and all the species inhabiting the Continent, — all, that is to say, 



