What kind of turtles are these?

My dad use to tell me when he was a child, he had these really small turtles and he said that never really lived long. Then two days ago on vacation, in the window of a store we saw an aquarium that was full of little turtles. He told me that was what he use to have. They are aquatic and their shells are just a little bit bigger the a quarter (they weren't babies or juvies)... Anybody got any idea what kind of turtles they are?