Fig. 216.—Characteristic Polynesian Mollusca: A, Achatinella vulpina Fér., Sandwich Is.; B, Partula planilabrum Pease, Society Is.
(a) The general features of the Polynesian province are very similar throughout, although the Mollusca of each island group are in the main peculiar. The species are mostly small and obscure. Helix scarcely occurs, its place being taken by small Zonitidae (Microcystis, Charopa, Trochomorpha, etc.), and by groups of so-called Patula (Endodonta, Pitys, etc.), the exact position of which is not yet settled. Libera, remarkable for its method of ovipositing (p. [128]), is peculiar to the Society and Hervey Is.; Partula is almost universal, attaining its maximum (40 sp.) in the Society Is.; Tornatellina, Pupa, and Vertigo occur throughout.
The land operculates consist chiefly of Omphalotropis, Pupina, Realia, and Helicina. Diplommatina and Palaina are abundant on the Pelews, and a Moussonia occurs in the Samoa Is. Ostodes, a small form of Cyclophorus, is found in some of the southern groups. The fresh-water operculates are Melania, Neritina (including Clithon, a sub-genus furnished with spines), and Navicella; there are no Unionidae, while fresh-water Pulmonata are very scarce.
(b) The land Mollusca of the Hawaiian province are distinguished by the possession of four entirely peculiar genera—Achatinella, Leptachatina, Carelia, and Auriculella. More than 300 of the two former genera have been described, every mountain valley of some of the islands having its own peculiar species. The destruction of the indigenous herbage by goats is rapidly extinguishing many forms. Partula, and the small land operculates, so characteristic of the other groups, are, with the exception of Helicina, entirely wanting. The occurrence of one of the Merope group of Helix (Solomon Is.) is remarkable, and there is a rich development of Succinea. “Patula,” Microcystis, Tornatellina, and the other small Polynesian land Pulmonata are well represented. The presence of Isidora, absent from the central Pacific groups, is remarkable, and Erinna is a peculiar genus belonging to the Limnaeidae.
CHAPTER XI
GEOGRAPHICAL DISTRIBUTION OF LAND MOLLUSCA (continued)—THE ETHIOPIAN, NEARCTIC, AND NEOTROPICAL REGIONS
D. The Ethiopian Region
The Ethiopian region includes the whole of Africa south of the Great Desert, and Southern Arabia, together with the outlying islands, excepting those of the Atlantidean province (p. [297]).
Regarded as a whole, the Ethiopian is poorest in land Mollusca of all the tropical regions. And yet its characteristics are very remarkable. The entire Achatina group is peculiar, and takes, especially in W. Africa, some curious forms (Columna, Perideris, Pseudachatina). Carnivorous Mollusca (Ennea, Gibbus, etc.) are highly developed, especially in the south and east, the largest known helicoid form (Aerope) being from Natal. In the possession of these types of the Agnatha, Africa is more closely related to the Australasian than to the Oriental region. The true Cyclostoma are entirely peculiar to the region, but are absent from West Africa.
Fresh-water Mollusca are abundant and characteristic, especially in and near the Great Lakes. Lanistes, Cleopatra, and Meladomus, among the operculates, together with Mutela and Aetheria (Unionidae), Galatea and Fischeria (Cyrenidae), are peculiar.
In its negative, as well as its positive features, the Ethiopian region is markedly isolated. Helicidae and Naninidae are equally deficient, the former, indeed, attaining some numerical predominance in the extreme south, but the species are nearly all insignificant in size and colouring. It is only in Madagascar that Helix asserts itself. Arion, Limax, Hyalinia, Clausilia, and a number of other genera abundant along the Mediterranean, are either altogether absent, or are very scantily represented. Land operculates, so characteristic of other tropical countries, are almost entirely wanting. If we disregard the Malagasy sub-region, there are scarcely forty species of land operculates on the whole African continent.