What Am I?

I found this shy reptile in the middle of the path on my reptile-rich hike Sunday in Back Bay National Wildlife Refuge.  See if you can figure out what it is, although I needed my field guide and the photos I took, so it won't be an easy one.  You will see photos at the end, but you will be trying to figure it out just from words (and maybe a field guide).

I have two means for my defense
To protect from foes quite intense

The first is wearing armor plate
So that dinner won't be my fate

And I can dive like submarine
If danger comes, I'll split the scene

For from water I'm never far
To dive and swim I am a star

Like periscope my head pokes out
All you will see is tip of snout

But I can see and breath quite fine
Security's my bottom line

I'm rather large, like dinner plate
My bright red markings are ornate

For on my shell are lines of red
(A color in my name, it's said)

And underneath, orangey-yellow
Smudged with gray, a handsome fellow

I love to bask at water's edge
To warm my blood up, I allege

Well, that is it
for my clues
take your guess
and scroll
D
O
W
N
to see what this animal is.

Red bellied turtle is correct
Hard to guess me, I do suspect

This animal may be confused for a painted turtle but it is much larger.  This is a female, and the males are marked differently.  Like many reptiles, there is a lot of color variation, and they can be quite dark.  She was very shy, and would not poke her head out far. 

I flipped her on her back for a couple of seconds to get a photo of her belly, then righted her and moved her off the path in the direction she had been heading - which was towards a body of water.  I don't know what she was doing out of water this time of year - maybe moving to a better spot to hibernate?