The genus Pachystyla (Naninidae) is quite peculiar, forming the main portion of the land snails proper. It attains its maximum in Mauritius (17 sp.), with 5 sp. in Bourbon and one sub-fossil sp. in Rodriguez, while in the Seychelles it is absent. But the principal feature of the Mascarene group is the extraordinary development of the carnivorous genus Gibbus, which has 27 sp. in Mauritius, 8 in Bourbon, 4 in Rodriguez; in the Seychelles, it is replaced by Edentulina and Streptostele. The principal link with Madagascar is found in a part of the operculate land fauna. Cyclostoma is present (with Otopoma) in several fine living forms, and the number of sub-fossil species is a clear indication that this group was, not long ago, much more abundant, for of the 16 Cyclostoma known from Mauritius 10 are sub-fossil. The operculates form a decided feature of the land fauna; thus in Mauritius there are 32 species, or more than 28 per cent of the whole.
Fig. 225.—Characteristic Mauritian land shells: A, Gibbus palanga Fér.; A´, young of same; B, Gibbus lyonetianus Pall.
Indian and Australasian affinities are unmistakably present. Thus Omphalotropis, a genus characteristic of small islands, is profusely represented, but it does not occur in Madagascar or Africa. Two Helicina (Mauritius and Seychelles) and a single Leptopoma (possibly a Leptopomoides) are also of eastern relationship. Cyclotopsis, Cyathopoma, and Geostilbia are markedly Indian genera. Microcystis, Patula, and Tornatellina are Polynesian. Hyalimax—and this is a very striking fact—occurs nowhere else but in the Andamans and Nicobars, and on the Aracan coast. The nearest relation to the Seychelles Mariaella appears to be the Cingalese Tennentia. Not a single representative of these eleven genera has been found even in Madagascar.
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.