982 



FRESH-WATER BIOLOGY 



31 (24) 



Shell small, turreted; spire somewhat elevated; spiral sculpture 

 wanting or subobsolete; columella not twisted; inner lip 

 usually reflected over the umbilicus. 



Subgenus Galba Schrank. 



This group of small species has a wide range from the Atlantic to the Pacific. 

 The example, Lymnaea obrussa Say, (Fig. 1425; X 12), is a common species in the 

 Eastern States. 



FIG. 1425. 



32 (50) Shell discoidal, sinistral, or dextral, or spiral with a very low spire. 



Animal sinistral; tentacles cylindrical. 



Family PLANORBIDAE . . 33 



The dextral species of this family present the apparent anomaly of a sinistral animal with a 

 dextrally-coiled shell. Such shells are not true dextral shells, but represent the condition of 

 hypertrophy, so called, in which the spiral growth of the shell, instead of being from the apex 

 downward, as is usually the case, is, as it were, from the apex upward, the result being an appar- 

 ently dextral shell with a sinistral animal. Such shells are also called ultradextral. In the 

 formation of the key to the subdivisions of the family, the shells are treated with reference to 

 their apparent mode of spiral growth. 



33 (47) Shell discoidal Subfamily PLANORBINAE . . 34 



34 (46) Aperture edentate Planorbis M tiller . . 35 



Six subgenera: 35, 36, 39, 40, 41, 42. 



35 (36) Shell sinistral, large; whorls rounded above and below, gradually in- 



creasing; aperture but slightly expanded; lip simple and 

 sharp. . Subgenus Planorbis s.s. 



A single, characteristic species, P. glabratus Say, (Fig. 1426; X i), is 

 found in Florida. 



FIG. 1426. 



36 (39) Shell dextral or sinistral, few whorled; the whorls carinate above and 



rapidly enlarging; base funicular; aperture suddenly ex- 

 panded and lip thickened. 



Subgenus Helisoma Swainson . . 37 



37 (3 8 ) Shell dextral, carinated above and below; spire and base funicular. 



Section Helisoma s.s. 



The typical species, Planorbis antrosus Con., (Fig. 1427; X i|),hasa 

 general distribution east of the Rocky Mountains and rarely on the 

 Pacific coast. 



FIG. 1427. 



