CHAP. XXII.] MOLLUSCA. 513 



to the limit of trees or even considerably beyond it, in the 

 Arctic regions ; but they are comparatively very scarce in all 

 cold countries. The Antilles, the Philippine Islands, Equa- 

 torial America, and the Mediterranean sub-region are especially 

 rich in this family. Comparatively few of the genera, and those 

 generally small ones, are restricted to single regions ; but on the 

 other hand very few are generally distributed, only two — Helix 

 and Pupa — occurring in all the six regions, while Helix alone is 

 truly cosmopolitan, occurring in every sub-region, in every 

 country, and perhaps in every island on the globe. 



The Neotropical region is, on the whole, the richest in this 

 family, the continental Equatorial districts producing an abun- 

 dance of large and handsome species, while the Antilles are 

 pre-eminent for the number of their peculiar forms. This 

 region possesses 22 of the genera, and 6 of them are peculiar. 



The Palsearctic region seems to come next in productiveness, 

 but this may be partly owing to its having been so thoroughly 

 explored. It possesses 16 of the genera, and 3 of them are 

 confined to it. The great mass of the species are found in 

 the warm and fertile countries surrounding the Mediterranean 

 Sea. 



The Ethiopian region has 13 genera, only one of which is 

 peculiar. 



The Australian region has 14 genera, 2 of which are confined 

 to the Pacific Islands. 



The Oriental has 15 genera and the Nearctic 12, but in 

 neither case are there any peculiar generic types. 



The following is the distribution of the several genera taken 

 in the order of their magnitude : — 



Helix (1,115 sp.), cosmopolitan. This genus is divided into 

 88 sub-genera, a number of which have a limited distribution. 

 An immense quantity of species have been recently described, 

 so that the number now exceeds 2,000. 



Nanina (290 sp.) is characteristic of the Oriental and Aus- 

 tralian regions, over the whole of which it extends, just entering 

 the Palsearctic region as far as North China and Japan. 

 Isolated from this area is a small group of 4 species occurring 



