This is an adult diamond backed terrapin, I have a photo of the little ones here http://www.flickr.com/photos/jbdrake/2793639880/
I find these little guys in my pool about twice a year.. This month I’ve found six! I had fun dropping them in the tadpole pool and watching them chase the little buggers mercilessly.
An interesting note.. Diamond backed Terrapins used to be considered the premier flavor for soups! they were harvested in huge numbers. Now I think that craze has calmed down but recent efforts to determine how many there are don’t look promising.
There are a few states that list them as protected species. Florida isn’t one of them.
The dining craze has given way to the pet trade… and occasionally I find ones that have very unusual colors and markings for this area. Like most reptiles around here I’m sure that a few unknowing people released some into the lagoon introducing those unusual genes.