340 



NORTH AMERICA 



but which spreads eastward into the Antilles. Among the 

 Limacidae, Limax is common to both sub-regions, l^ut Teheniio- 

 2)horus (4 sp., o 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 3Iesomphix and Gastrodonta (peculiar 

 to this sub-region), and Hyalinia proper. Patula is well re- 

 presented. The Helicidae belong principally to the groups 

 Mesodon, Stenotrema, Triodopsis, Folygyra, and Strohila, 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). 



The poverty of the land fauna is atoned for by the extra- 

 ordinary abundance and variety of the fresh-water genera. A 

 family of operculates, the Pleuroceridae, with 10 genera and 



Fig. 22G. — Characteristic 

 North American Mol- 

 lu.sca. A, Helix [Me- 

 sodon] palliata Say, 

 Ohio. B, Helix [Poly- 

 <jyra) cereolus Miihlf., 

 Texas. C, Patula al- 

 ternata Say, Tennessee. 

 ABC 



about 450 species, is quite peculiar, a few stragglers only reach- 

 ing Central America and the Antilles. The nucleus of their 

 distribution is the Upper Tennessee Elver with its branches, and 

 the Coosa Eiver. They appear to dislike the neighbourhood of 

 the sea, and are never fomul 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 Eiver, 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 Eiver, the Mississippi, the Chattahoochee Eiver, and 

 the Gulf of Mexico. The Mississippi is a formidable barrier to 

 their extension, and a whole section {Tryixmostoma, with the 

 four genera lo, Pleurocera, Angitrenia, 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 I'ulmouata are also abundant, while the richness 



