I agree with DiscusMel that it seems to be a diamond back terrapin (Malaclemys terrapin) but am unsure which subspecies, a picture of the face and plastron can help find that out.
All the subspecies that I know of are similar in size and habits, males to 5" carapace, females to 9". They eat mainly snails, clam, crayfish, shrimp, and crabs, but also eat small fish and amphibs if they can catch them. They prefer brackish water and tend to get skin fungus if they don't get it at least as the occasional bath.
