THE MOLLUSCA 



989 



72 (90) Shell small, spiral, dextral. Central tooth of the radula with one or 



more basal denticles. . . . Family Amxicolidak . . 73 

 Five subfamilies: 73, 74, 82. 88, 89. 



73 (74) Operculum calcareous, concentric Subfamily Bythininae. 



Only one genus Bythinia Gray. 



A sinRle European species, B. tentaculala L., (Fig. 1454; X 2), has been intro- 

 duced by commerce from the Hudson River, along the line of the Erie Canal, 

 and into the Great Lakes as far west as Chicago. 



Fig. 1454- 



74 (82) Operculum corneus, paucispiral. Shell thin; columella not thickened. 



Foot simple Subfamily Amnicolinae . . 75 



Six genera: 75, 77, 78, 79, 80, 81. 



75 (76) Shell smooth, usually subglobose. Central tooth of the radula multi- 



cuspid, with a tongue-shaped process projecting on the ante- 

 rior surface and beyond the base and with several basal 

 denticles Amnicola Gould and Haldeman . . 76 



Very numerous both in species and individuals. In shell characters some of 

 the more elongate species approach Paludestrina, but as a rule the shell is more 

 globose, with a shorter spire. Type, A. limosa Say, (Fig. 1455; X 4)- 



Radula more minute and the denticulations of the cusps of the teeth 

 finer and sharper Subgenus Cincinnatta Pilsbry. 



This subdivision is based wholly on the character of the lingual teeth. The 

 shell characters are those of Amnicola. Type, Amnicola cmcinnatiensis Anth., 

 (Fig. 1456; X2i). 



Shell similar to Amnicola, but more slender and elongated. Central 

 tooth of the radula with but one basal denticle on each side 

 and without the tongue-shaped process. 



Paludestrina Orbigny. 



The species of this genus are numerous and range from the Atlantic to the Pa- 

 cific. Example, P. nickliniana Lea, (Fig. i457; X 6). 



Shell elongated, turreted, longitudinally ribbed or pHcate. 



Trvonia Stimpson. 



Fig. 14s 5 



76 



Fig. 1456. 



77 (78) 



Fig. 1457- 

 78 (77) 



The type and only species, T. ./a//,ra/. Stimp.. (Fig. 145S; X 2i), is found fossU 

 M W in southern California and living in Nevada. 



Fig. 1458. 



