How do you tell the sex of a box turtle?

I've loved turtles and terrapins all my life. This is what I've learned, and I hope it helps you... Typically, the girl turtle's shell is flat on the bottom. A boy turtle's shell has a concave (bowl shape)on the bottom. This shape comes in handy when turtles mate, as the bottom of the boy turtle's shell must fit comfortably over the back portion of the girl turtle's shell. So, girls have a flat bottomed shell & boys have a "bowl"(or concave)shape on the bottom.