The entire Mississippi basin is inhabited by a common assemblage of Unionidae, and a considerable number of the species are distributed over the whole of this area, Texas, and parts of E. Mexico. Some species have spread out of this area into Michigan, Canada, the Red River, and Hudson’s Bay district, and even into streams in New York which drain into the Atlantic. An entirely different set of forms occupy the great majority of the rivers falling into the Atlantic, the Appalachian Mountains acting as an effective barrier between the two groups of species, which appear to mingle below the southern end of the range. In many cases Unionidae seem to have no difficulty in migrating from river to river, if the distance is not extreme; they probably are carried across overflowed districts in time of flood.[378] [Illustration: FIG. 227.--_Helix_ (_Arionta_) _fidelis_ Gray, Oregon.] (2) =The Californian Sub-region= is markedly distinct from the rest of N. America. The characteristic sombre Helices of the Eastern States are almost entirely wanting, and are replaced by _Arionta_ (20 sp.), a larger and more varied group, which may have some affinity to Chinese forms. _Glyptostoma_ (1 sp.) is also peculiar. _Selenites_ here has its metropolis, and _Pristiolma_ is a remarkable group of small _Hyalinia_ (_Zonites_), but the larger forms of the Eastern States are wanting. Several remarkable and quite peculiar forms of slug occur, namely, _Ariolimax_ (whose nearest relation is _Arion_), _Prophysaon_, _Hemphillia_, and _Binneya_. There are no land operculates. Not more than 15 to 20 species of the Pleuroceridae (sect. _Goniobasis_) occur west of the Rocky Mountains, and only a single _Unio_, 5 _Anodonta_, and 1 _Margaritana_, which is common to New England. _Pompholyx_ is a very remarkable ultra-dextral form of _Limnaea_, apparently akin to the _Choanomphalus_ of L. Baikal. _Bithynia_, absent from the Eastern States, is represented by two species. The general indications are in favour of the Californian fauna having migrated from an Old World source after the upheaval of the Sierras; the American fauna, on the other hand, is purely indigenous, with no recent Old World influence at all. _Land Mollusca of the Nearctic Region_