(1) The N. American Sub-region.—The Carnivorous genera are represented solely by the few Glandina mentioned above, and by the indigenous genus Selenites, a form midway between Testacella and Limax, whose metropolis is on the Pacific slope, but which spreads eastward into the Antilles. Among the Limacidae, Limax is common to both sub-regions, but Tebennophorus (4 sp., 3 of which belong to the genus Pallifera), a genus found also in China and Siam, and Vitrinozonites do not occur in the Californian. Hyalinia (Zonites) is fairly abundant, especially in the groups Mesomphix and Gastrodonta (peculiar to this sub-region), and Hyalinia proper. Patula is well represented. The Helicidae belong principally to the groups Mesodon, Stenotrema, Triodopsis, Polygyra, and Strobila, only 6 of which, out of a total of 84, reach the Pacific slope. Land operculates are conspicuous for their almost complete absence (see map, frontispiece).
Fig. 226.—Characteristic North American Mollusca. A, Helix (Mesodon) palliata Say, Ohio. B, Helix (Polygyra) cereolus Mühlf., Texas. C, Patula alternata Say, Tennessee.
The poverty of the land fauna is atoned for by the extraordinary abundance and variety of the fresh-water genera. A family of operculates, the Pleuroceridae, with 10 genera and about 450 species, is quite peculiar, a few stragglers only reaching Central America and the Antilles. The nucleus of their distribution is the Upper Tennessee River with its branches, and the Coosa River. They appear to dislike the neighbourhood of the sea, and are never found numerously within 100 miles of it. They adhere to stones in rapid water, and differ from the Melaniidae of the Old World and of S. America in the absence of a fringe to the mantle and in being oviparous. They do not occur north of the St. Lawrence River, or north of U.S. territory in the west, or in New England. Three-quarters of all the known species inhabit the rough square formed by the Tennessee River, the Mississippi, the Chattahoochee River, and the Gulf of Mexico. The Mississippi is a formidable barrier to their extension, and a whole section (Trypanostoma, with the four genera Io, Pleurocera, Angitrema, and Lithasia) does not occur west of that river. The Viviparidae are also very largely developed, the genera Melantho, Lioplax, and Tulotoma being peculiar. The Pulmonata are also abundant, while the richness of the Unionidae may be gathered from the fact that Wetherby states[377] that in 1874 no less than 832 species in all had been described.
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]
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