It's a...

Girl!!
Mason already calls her " Baby Sis."
And here are the pictures...
(I have added captions to help explain what is what)

Here is her little arm. The elbow is to the right and her hand is over her face.

Here is a good shot of her spine. Her bum is down on the right side.

Spine shot along with her head on the left.

Look at that big ole' foot! She's probably going to be taller than both Reese and I if she ever grows into those feet. Her other leg is crossed over her crotch. Her left knee is to the right of left big toe, and her bum is to the right of her shin. Got it?

Here is a picture of her right leg. Her foot is to the right and her knee is bent. Her legs were crossed and tucked right up under her the whole time so it wasn't until the very end of the ultrasound that we were able to find out that she was in fact a girl. I have the actual picture showing that she is a girl but she was so modest and not wanting to show us that I figure I won't post her exposure on the internet. :)

She was also very camera shy. The arrow is pointing to her upper lip, her nose is to the left and her bottom lip and chin are to the right. This is the best picture we were able to get of her little face. So, like her brother, they both didn't want to show off their cute little faces. We have some 3D images of her face that help you to actually see her face better but they didn't get onto the CD that we have. Here is the picture we have of Mason's face in his ultrasound. They are already alike, only showing their noses and mouths.

So did you guess right...boy or girl?