Turns out my newest visitor is a Peninsula Cooter, and Jim says the shorter claws indicate she is a she (males have longer nails to hold on to the females during mating). She was about the size of a large dinner plate, and Jim guessed her to be about 12 years old.
I showed him where I had found her, and he pointed out that she had dug a nest hole and was preparing to lay eggs when I moved her to the courtyard. I felt bad about that, but I feared some of the large dogs frequently walked off-leash in the area would attack her and knew she'd be safer in my courtyard. (We checked - no eggs in the hole.)
As luck would have it, Jim had this dandy photo of a baby Peninsula Cooter. Awwww! (Be sure to click on all the photos to enlarge.)
After a suitable photo op, we drove her to a nice pond where Jim says other cooters spend their days sunning. This was so cool - as soon as we got her out of the car and she saw/smelled the pond, she got all excited! Jim carried her about half way down to the water's edge and set her down. As I was chastising him for not putting her nearer the water, she hauled turtle butt as fast as she could - my old camera missed a final shot of her as she slid into the water, but I guarantee you that she was smiling.
A good day.
(All photos by Jim Angy except the dirt and this last one, which I took)