Mixed Stack of Turtles at Zoo Simon Bolivar, 12/04/14

large turtles with black heads are Rhinoclemmys funerea.

Agreed - and the slider back right I think is Trachemys venusta; top-middle slider I think is T. s. elegans (it seems to be the same individual as the one on the sign with a slightly malformed carapace).

The turtle pool had a label showing:

Rhinoclemmys funerea
Rhinoclemmys pulcherrima
Trachemys venusta
Trachemys scripta elegans
Trachemys emolli
Kinosternon leucosternum
Kinosternon scorpioides
Chelydra acutirostris

Not sure if all of these were still present as I was pushed for time and couldn't spend too long checking! Will post a photo of the sign in due course.