I'm not good with these basking turtles, but I can tell you that it's a cooter of some sort. Where is it from? Nowadays, with people releasing pet turtles in areas where they're not native you will see a lot of hybridization. Giving the location would help someone nail down the exact species. I can tell you one thing, this turtle looks sick. They're usually very energetic little guys. You might want to contact a vet that's experienced with turtles.