493 
Lymnezas have been described which at first sight appear quite 
distinct. These are Lymnea reflexa Say, Lymnea reflexa crystalensis 
Baker, and Lymnea palustris michiganensis Walker. The summer-dry 
ponds studied yield all three forms, and it is at once apparent that 
they represent age variation only, michiganensis being quite imma- 
ture (figures 7-8), crystalensis being three-quarters grown (figures 
2-3), and reflexa representing the fully mature mollusk (figure 1). 
The Oregon and Washington forms cited by Walker (Nautilus, VI, 
p. 33) are probably the young of Lymnea proxima rowellii, with which 
reflexa has been confused by western conchologists. The figures on 
the plate indicate the age variation. 
Family PLANORBIDA. 
Genus PLANorBIs Miiller. 
Subgenus Helisoma Swainson. 
Planorbis trivoluis Say. Moll. Chi. Area, Pl. XXXII, Fig. 7-10. 
mearions Tift, VI, VII, 1X, XX, XXII, XXXII, XXXIII, 
XXXV, XXXVI. 
Subgenus Hippeutis Agassiz. 
Planorbis exacuous Say. Moll. Chi. Area, Pl. XXVI, Fig. 5. 
Stations XIII, XVI. 
Subgenus Gyraulus Agassiz. 
Planorbis parvus Say. Moll. Chi. Area, Pl. X XVI, Fig. 7. 
Stations IV, X XI. 
Genus SEGMENTINA Fleming. 
Subgenus Planorbula Haldeman. 
Segmentina armigera (Say). Moll. Chi. Area, Pl. XXX, Fig. 32. 
stations Ii TV; Vil, ix, x XI, XIV, XVI, XXXII. 
Family ANCYLIDA. 
Genus AncyLus Geoffroy. 
Subgenus Ferrissia Walker. 
Ancylus rwularis Say. Moll. Chi. Area, Pl. XXX, Fig. 29. 
Station X XI. 
Ancylus parallelus Haldeman. Pl. XXV, Fig. 22. 
Station XXXII. 
