What kind of turtle is this?

Okay i found a turtle over a year ago. im still not sure what kind of turtle it is...he's black and has orange mottleing on the undersude of his shell and orange stripes around the ring (endge) of the carapace. he is completely aquatic and has a sharp mouth/beak tip.he has a grey head and neck with no designs on his skin....so could anyone tell me what kind of turtle he is?