Definitely Juglans. Looks like a Juglans nigra nut, but could be one of it's smaller-nut relatives.

This post was edited by brandon7 on Tue, Nov 19, 13 at 1:06

Agree. Juglans.
There are some J.nigra that make a pretty small nut, but I'd be thinking J.microcarpa or possibly J.major.

I've had some microcarpa nuts in the past, and they can vary from dime to quarter size...

Hmm, how do you know it's not a Black Hickory (Carya texana) - and what's the difference?

Here is a link that might be useful: Black Hickory?

hickory nuts are slightly oblong and and have seams like a football. I would agree it is a walnut of some sort