_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. [Illustration: 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_.