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CLass GASTROPODA. 
SUBCLASS EUTHYNEURA. 
OrDER PULMONATA. 
SUBORDER BASOMMATOPHORA. 
Superfamily HyGROPHILA. 
Family PHYSID. 
Genus Puysa Draparnaud. 
Physa gyrina Say. Pl. XXV, Fig. 9-17. 
Stations I, IT, III, IV, V1, VII, 1X, X, XI, XIV, XV, XV eae 
XIX, XX, XXII; XXII, AXITT, A XV, KX, XKKV eae 
Physa gyrina is characteristic of nearly all the aquatic habitats of 
the area. A study of the material, collected at different seasons, 
between April and November, shows that two species have been 
founded on age variation. The smaller forms (Pl. XXV, Fig. 14-17) 
are Tryon’s oleacea and are immature. The larger forms (Pl. XXV, 
Fig. 9-13) are mature and represent Say’s gyrina. ‘The figures illus- 
trate well the degree of elongation of the spire as the shell matures. 
The two forms were always found associated together. This dis- 
covery illustrates the value of ecological studies. 
Genus APLEXA Fleming. 
Aplexa hypnorum (Linné). Moll. Chi. Area, Pl. XXXII, Fig. 16. 
Stations IV, XI, XIII, XV, XXVII, XXXI. HAypnorum is usually 
found in transient ponds or pools, associated with Lymnea caperata 
and Spherium occidentale. 
Family LYMNAIDA. 
Subfamily LYMNAINZA. 
Genus LyMN@&A Lamarck. 
Subgenus Galba Schrank. 
Lymnea caperata Say. Moll. Chi. Area, Pl. XXX, Fig. 18. 
Stations IV, XI, XIII, XV, XIX, XXV, XXVI, XXXV. 
Lymnea parva sterkiut Baker. Pl. XXV, Fig. 21. 
Stations IV, XXII. 
Subgenus Stagnicola Leach. 
Lymnea reflexa Say. Pl. XXV, Fig. 1-8. 
Stations I, II, III, VII, 1X, XI, XIV, XVI, XVIII. The study of 
this species has revealed some very interesting results. Three forms of 
