Wikipedia
Tryonia
Tryonia is a genus of freshwater snails in the family Hydrobiidae.This genus is sometimes placed in the family Cochliopidae.
Snails of this genus are very small with narrow shells. Females brood young within the genital tract. Species usually live in springs. The genus is differentiated from others by the structure of the male and female reproductive systems.
Most of these snails occur in western North America, especially the Chihuahuan Desert. There are also species known in Florida and Guatemala.