Chester gastropods from southern Illinois, eastern Missouri, and north-western Kentucky are described. The method used in discriminating the immature from adult individuals is explained. Fifty-three named species, 20 indetermined species and four variants are described. Four new genera are as follows: Yochelsonospira, Wellergyi, Kinkaidia, Globozyga. -- Thirty-eight new species are as follows: Bellerophon (Bellerophon) claxtonensis -- B. (B.) menardensis -- Knightites (Cymatospira) welleri -- Patellilabia chesterensis -- Straparollus (Euomphalus) illinoisensis -- Trepospira (Trepospira) chesterensis -- T. (Angyomphalus) kentuckiensis -- Balyea okawensis -- Euconospira sturgeoni -- Neilsonia welleri -- Wellergyi chesterensis -- Yochelsonospira pagoda -- Gosseletina johnsoni -- Porcellia chesterensis -- Glyptotomaria (Dictyotomaria) yochelsoni -- Phymatopleura nyi -- Kinkaidia cancellata -- Strophostylus chesterensis -- Naticopsis (Naticopsis) mariona -- Aclisina golconda -- A. marvinwelleri -- Stegocoelia (Stegocoelia) kentuckiensis -- S. (Stegocoelia) okawensis -- S. (Hypergonia) baldwinensis -- Platyzona americana -- Eotrochus marigoldensis -- Paleozygopleura venusta -- P. welleri -- Bulimorpha minor -- Leptoptygma golconda -- Globozyga tenuistriata -- Meekospira bambooformis -- M. batteni -- M. evansvillensis -- M. mississippiensis -- Donaldina marigoldensis -- D. americana -- D. zadoe.