The Caspian Sea, like Lakes Baikal and Tanganyika, is distinguished by the possession of several remarkable and peculiar genera. The sea itself, the waters of which are brackish, is 80 feet below the level of the Black Sea, and is no doubt a relict of what formed, in earlier times, a very much larger expanse of water. Marine deposits containing fauna now characteristic of the Caspian, have been found as far north as the Samara bend of the Volga. It is probable, therefore, that in Post-pliocene times an arm of the Aralo-Caspian Sea penetrated northward up the present basin of the Volga to at least 54° N. The Kazan depression of the Volga (55° N.) also contains characteristic Caspian fossils.[367] According to Brusina,[368] the Caspian fauna, as a whole, is closely related to the Tertiary fauna of southern Europe. Twenty-six species of univalve Mollusca, the majority being modified forms of _Hydrobia_, have been described from the Caspian, namely, _Micromelania_ (6), _Caspia_ (7), _Clessinia_ (3), _Nematurella_ (3), _Lithoglyphus_ (1), _Planorbis_ (1), _Zagrabica_ (1), _Hydrobia_ (2), _Neritina_ (2). The bivalves are mostly modified forms of _Cardium_ (_Didacna_, _Adacna_, _Monodacna_), which also occur in estuaries along the north of the Black Sea. A form of _Cardium edule_ itself occurs, and numberless varieties of the same species are found in a semi-fossil condition in the dry or half dry lake-beds, which are so abundant throughout the Aral district. (_d_) _The Atlantidean province_ consists of the four groups of islands, the Madeiran group, the Canaries, the Azores, and the Cape Verdes. _The Madeiran group_ contains between 140 and 150 species of Mollusca which may be regarded as indigenous, the great majority of which are peculiar. Only 11 species are common to Madeira and to the Azores, and about the same number, in spite of their much greater proximity, to Madeira and the Canaries. No less than 74 species, or almost exactly one-half, belong to _Helix_, and 9 to _Patula_. A considerable number of the Helices are not only specifically but generically peculiar, the genera bearing close relationship to those occurring in the Mediterranean region. As a rule they are small in size, but often of singular beauty of ornamentation. Various forms of _Pupa_ are exceedingly abundant (28 sp.), as is also _Ferussacia_ (12 sp.). There are also 3 _Clausilia_ (which genus occurs on this group alone), and 3 _Vitrina_ (a genus which occurs on all the groups). The land operculates are represented solely by 4 _Craspedopoma_, which is common to all the groups except the Cape Verdes. [Illustration: FIG. 201.--Characteristic land Mollusca from the Madeira group: =A=, _Helix_ (_Irus_) _laciniosa_ Lowe, Madeira; =B=, _Helix_ (_Hystricella_) _turricula_ Lowe, Porto Santo; =C=, _Helix_ (_Iberus_) _Wollastoni_ Lowe, Porto Santo; =D=, _Helix_ (_Coronaria_) _delphinuloides_ Lowe, Madeira.]