What does concave and convex mean?
Concave means that the object curves inward (it has a hollow).
Convex means that the object curves outward (it has a mound).
A tortoise is a land reptile with an exoskeleton, which is a hard shell.
In fact, there is a hard shell on the top (called a carapace) and a hard shell underneath (called a plastron).
How can you tell a male tortoise from a female tortoise? One way to tell the difference between a male and a female tortoise is to turn it around and look at the plastron – the underbelly shell.