Lake Baikal.—The Mollusca of Lake Baikal exhibit distinct characteristics of their own, which seem to indicate the long-continued existence of the lake in its present condition. Several entirely peculiar genera occur, which are specialised forms of Hydrobia, e.g. Baikalia, Liobaikalia, Gerstfeldtia, Dybowskia, and Maackia; Benedictia alone extends to the basin of the Amoor. Choanomphalus, another peculiar and ultra-dextral (p. [250]) genus belonging to the Limnaeidae, appears to be related to the West American Carinifex.
(2) The Mediterranean Sub-region is divided into four provinces: (a) The Mediterranean province proper; (b) the Pontic; (c) the Caucasian; and (d) the Atlantidean province.
(a) The Mediterranean province proper is best considered by further subdividing it, with Fischer and others, into separate districts, each of which has certain peculiar characteristics.
(i) The Hispano-Algerian district includes the greater part of the Iberian peninsula, the Balearic Islands, and northern Africa from Morocco to Tunis. The extreme western parts of these districts, including West Morocco, Portugal, Asturias, and south-west France, under the influence of the moist climate caused by the Atlantic, show some peculiar features which, in the view of some, are sufficient to justify their separation from the rest of the Hispano-Algerian portion. Among these is a marked development of the slugs, Testacella, Arion, and Geomalacus, the latter of which occurs even in south-western Ireland.
Fig. 194.—A, Parmacella Valenciensii W. and B. × ⅔. (After Moquin-Tandon.) A´, shell of the same, natural size.
Spain.—The principal features are the development of the Macularia, Iberus, and Gonostoma groups of Helix, and the occurrence of the remarkable slug Parmacella, which is found in many other parts of the sub-region, and extends eastward as far as Afghanistan. Clausilia has but few species, mostly in the north. There are four species of land operculates, one of which is referred to a genus (Tudora) now living only in the West Indies, but which occurs in the Eocene fossils of the Paris basin. In the south there are several species of Melanopsis and Neritina.
The States of Northern Africa have a thoroughly Mediterranean fauna, whose facies on the whole shows rather more affinity to Spain than to Sicily. The Helices of Morocco and Algeria belong to the same groups as those of southern Spain. Many are of a dead white colour, the better to resist the scorching effect of the sun. Ferussacia is abundant, Geomalacus and Parmacella are represented by a single species each, and there is one Clausilia. According to Kobelt,[366] the original land connexion between southern Spain and Morocco must have been much more extensive than is usually assumed, and probably reached at least to the meridian of Oran and Cartagena. The Mollusca of Oran and Cartagena are, according to him, much more closely related than those of Oran and Tangier, or those of Cartagena and Gibraltar, but at Cartagena some species, which are characteristic of the Mediterranean coasts from Syria westward, disappear, are absent from the rest of Spain and from Morocco, but reappear on the south-western coasts of France. These species may possibly have pushed along that arm of the sea which, when the Straits of Gibraltar were closed as far as the latitude of Oran and Cartagena, united in comparatively recent times the Bay of Biscay with the Gulf of Lions.
The following genera, which do not occur in Spain, have probably spread into northern Africa as far as Algeria, via Sicily and Tunis, namely, Glandina (1 sp.), Daudebardia (1 sp.), Pomatias (2 sp.). Tunis shows strong traces of Sicilian influence, and Kobelt found a colony of snails, of Sicilian affinities, as far west as Tetuan.
The Sahara.—The Algerian Sahara contains, in many places, a sub-fossil Molluscan fauna which appears to show that the district has, in quite recent times, undergone a gradual desiccation. The species are mainly fresh-water, including Melania, Melanopsis, and Corbicula, with here and there valves of Cardium edule, and indicate, on the whole, an affinity with recent Egyptian, rather than North African species. It is probable that a vast series of étangs, or brackish-water lakes, once stretched along this region, and were ultimately connected with the sea somewhere between Tunis and Egypt.