Family Endodontidm 
G4. Genus Pyramidula 
8. Shell large, depressed, thin; aperture simple; 
strong rib-like striae; wide umbilicus P. alternata 
G5. Genus Helicodiscus 
9. Shell small, discoidal, widely umbilicated ; 
covered by parallel, equidistant, revolving 
lines H. lineatus 
d. Thin, almost transparent, aperture very large in proportion to the 
small, short spire, whorls rapidly expanding 
Family Succinidcs 
G6. Genus Succinea 
10. Shell large, thin, fragile; aperture occupy- 
ing over two-thirds of the shell S. ovalis 
e. Spire elevated and dextral, or flat; some sinistral 
Family Lymneida 
G7. Spire elevated and dextral 
Genus Lymnaea 
11. Shell thin, high, large, whorls rapidly ex- 
panding L. stagnalis . appressa 
12. Shells stout, solid, small, shouldered whorls 
• and small aperture narrow at the top L. obrussa decampi 
13. Shell stout, inflated, shouldered whorls and 
rounded aperture L. galbana 
14. Shell solid, high, subconic, long pointed apex 
and elongate-ovate aperture L. obrussa 
G8. Spire flat 
Genus Planorbis 
15. Shell sinistral, with funnel-shaped umbilicus 
and depressed spire, whorls carinate above 
and below P. antrosus 
16. Shell sinistral with flat spire; small um- 
bilicus and pronounced bell-shaped expan- 
sion of last whorl behind the aperture . P. campanulatus 
17. Shell small, up to 4 mm. in diameter, dex- 
tral, flat above; wide, shallow umbilicus 
below, periphery of whorls rounded P. altissimus 
18. Shell small, dextral, flat above, last whorl 
deflected downward, periphery of whorls 
subcarinate P . defiectus 
f. Shell elevated and sinistral 
