Turtles

On Saturday, a day of glorious spring, the Japanese Hill and Pond Garden at the BBG was thick with turtles soaking up the heat of the sun. Being cold blooded, they really need that heat. On the other hand, they can usually survive the winter, and freezing water, quite nicely. Some bury themselves in the mud and murk below the freezing line. They slow way down and stop breathing with their lungs. They can be in this state for three-four months, even half a year.
Most of our urban turtles are red-eared sliders, an invasive, released via the turtle trade and idiot pet owners species. But do you notice something different with the big one in these top two pictures? The one with algae-covered carapace? It isn’t one of the ubiquitous red-eared. But what species is it?

For what I understand, the red mark behind the ear on the sliders can fade with age. However, I’m looking at the shape of the shell and also the patterns on the skin around the neck (hard to see in the photos, but it looks more circular). My Peterson Field Guide is no help; the illustrations are way too small.
Meanwhile, it’s always about real estate in this town, isn’t it?