The fresh-water Mollusca (omitting the Neritidae) are: Mauritius 9 species, Bourbon 5, Rodriguez 4, Seychelles 6, with only 15 species in all. The one _Planorbis_ and the _Vivipara_, the _Paludomus_ and two of the _Melania_ are of Indian types. The _Lantzia_ (peculiar to Bourbon) is probably allied to the Indian _Camptonyx_. Owing to the paucity of permanent streams, no fresh-water bivalves occur. Among the Neritidae is a single _Septaria_, a genus which, though occurring in Madagascar, is entirely strange to Africa, and is abundant in the Oriental and Australasian regions. It would seem probable that when the closer connexion which at one time undoubtedly existed between India and Eastern Africa began to be less continuous,[376] the Mascarene group was first severed from what ultimately became Madagascar, while the Seychelles, and perhaps the Comoros, still continued united to it. The Comoros, which lack the great _Helices_, separated off from Madagascar first, while the Seychelles continued in more or less direct union with that island sufficiently long to receive the progenitors of _Stylodonta_ (a peculiar group of _Helix_), but became disunited at an exceedingly remote period. =E. The Nearctic Region= The southern boundary of this region may be regarded as roughly corresponding to that of the United States, _i.e._ Lower California and Mexico are excluded. The southern portion of Florida belongs to the Antillean sub-region. The principal characteristic of the Nearctic Region is the remarkable poverty of its land Mollusca. No district in the world of equal extent is so poor in genera, while those which occur are generally of small size, with scarcely anything remarkable either in colouring or form. The elongated land shells (_Clausilia_, _Buliminus_), so characteristic of Europe, are entirely wanting, but a few _Bulimulus_, of Neotropical origin, penetrate Texas, and from the same sources come a few species of _Glandina_ (as far north as S. Carolina), _Holospira_ (Texas), and _Helicina_. The region falls into two well-marked sub-regions, the N. American and the Californian, with the Rocky Mountain district as a sort of debatable ground between them. The Californian sub-region consists of the narrow strip of country between the Sierra Nevada, the Cascade Mountains and the coast-line, from San Diego to Alaska; the N. American sub-region consists of the remainder of the region.