;40 



NORTH AMERICA 



CHAP. 



but which spreads eastward into the Antilles. Among the 

 Limacidae, Limax is common to both sub-regions, but Tehenyio- 

 phorus (4 sp., 3 of which belong to the genus PalUferd)^ a 

 genus found also in China and Siam, and Vitrinozonites do not 

 occur in the Californian. Hyalinia QZonites) is fairly abundant, 

 especially in the groups Mesompliix and G-astrodonta (peculiar 

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

 presented. The Helicidae belong principally to the groups 

 Mesodon, Stenotrema, Triodopsis^ Polygyra^ and Strohila^ o^^ly 6 

 of which, out of a total of 84, reach the Pacific sIo^dc. Land 

 operculates are conspicuous for their almost complete absence 

 (see vii'^^^ frontispiece) . 



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

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

 family of operculates, the Pleuroeeridae, with 10 genera and 



Fig. 226. — Characteristic 

 North American Mol- 

 lusca. A, Helix {Me- 

 sodon) paUktta Say, 

 Ohio. B, Helix {Poly- 

 gyra) cereolus Miihlf., 

 Texas. C, Patula al- 

 ternata Say, Tennessee. 



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 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 IVIississippi, 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 Jo, Pleurocera^ Angitrema, and Lithasia') does not 

 occur west of that river. The Viviparidae are also very largely 

 develo[)ed, the genera Melantlio^ Tiioplax^ and Tnlotoma being 

 peculiar. The Pulmonata are also abundant, while the richness 



