Pig - nut hickory I think. However, if the nut is not bitter, it may be shag bark hickory

http://www.oplin.lib.oh.us/products/tree/fact%20pages/hickory_pignut/hickory_pignut.html

The bitternut hickory is often called the pignut, but it is a small and bitter nut -- the squirels just let them lay in my yard. This tree is actually the tallest tree found in a lot of the midwest areas.
I bet you are referring to the shellbark hickory, which is a shag bark hickory, but larger in size nut. These tend to go fast when they ripen, both animals and people being eager to get so much sweet meat with so little work.
Jeff

There is a sweet pignut hickory, Carya ovalis or Carya glabra var. odorata, also called red hickory. It may be a hybrid between C. glabra (pignut) and C. ovata (shagbark). Pignut can also hybridize with bitternut. Bitternut, butternut, or yellowbud hickory are names usually given to Carya cordiformis. Pecan can be considered a hickory since it's in the same genus. It doesn't seem to bear fruit in this zone even though it can be grown here.

Is the husk very thin (1/16")? That's Pignut or Bitternut, but almost never taste good. Thicker husks (1/8 - 1/4") would be Mockernut, Shagbark, Shellbark, or some hybrid.

Euell Gibbons ued to day that if you boil or soak pignut hickories in frequent changes of boiling water, it would leach out the tannin. We tried it once, and added ten changes of water before we gave up. They were still bitted.

Hickery nuts very good.Every year my family and i go up in the woods to a a clearing that has hundreds of them. Ive always loved them, even as a little kid. :)
jill

I have both pig nut hickory and shagbark hickory in my yard. I've noticed that the pig nut is generally smaller in size than the shagbark. I've never tried eating either although my uncle loves the shagbark nuts. The trees are almost indistinguishable except for the shagbark shedding all over my lawn. I also have butternut and black walnut trees. A lot of people would tell me I'm lucky but then I would invite them to come over in October and walk through my yard without twisting an ankle. Ha Ha.

The squirrels don't eat them, that's how bitter they are. That's a good clue for us, I think,

I too grew up in Massachusetts. We always picked the pignut trees in the yard.
I love pignuts they are very good after they have a had a chance to dry under the bed in a box. It seems we always had lots of nuts to eat around Christmas My dad alwayspicked them and he has been gone for 10 year. = ( I wanted to plant trees in my own yard in Vermont but I have not looked into this yet. Those of you who say this nut is bitter, did you really try the nut? As a kid I would eat the green nut, just because. It was gooey and not really appealing but I will still have to crack one open with a rock and peal the not ripe meat out just for old times sake. Pig Nuts Rock! So does the memory
Loving the pignuts and looking to go and pick some in the following weeks.