I could not see the color of the belly since Mr. Turtle was sunbathing. This may be the wrong classification for him but I thought it looked most like the pond slider.
Locals insisted this species was venomous. They often kill them when they encounter them on their farms and ranches. When I told them it wasn't venomous, one man said, "well, no, it's not venomous, but the bite does cause cancer."