(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_). [Illustration: 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.