What are the carapace and the plastron?

plastron, a nearly flat part of the shell structure of a turtle or tortoise, what one would call the belly

plastron, a nearly flat part of the shell structure of a turtle or tortoise, what one would call the belly

It only applies to male turtles. They have a concave down bottom shell to make mating easier.

The plastron is the part of the shell that is on the belly of the turtle. The higher domed part is called the carapace.