A tall nut tree

There's a tree I walk by on my way to work every day.  The branches are all very high, so I can't get much of a look at the leaves.  But this is that time of year they call "fall," which means the leaves fall down to where I can get a better look at them.  Here are some photos:

Two from September 16:

One from October 4:

So we know we've got pinnately compound leaves, and The Tree Identification Book suggests pecan, hickory, walnut, ash, ailanthus, sumac, mountain ash, and box elder.  Now I am acquainted with ash, ailanthus, sumac, and box elder, and this is not one of those.  Another clue is that it has nuts.  They look kind of like tennis balls, with the bright greenish color, though they are a little smaller than tennis balls.  Mountain ash is not a nut tree, so that leaves us with pecan, hickory, and walnut.  The Sibley Guide to Trees tells us that these are related kinds of trees, in family Juglandaceae.  

So, which kind could it be?  The top picture shows 11 leaflets on a leaf, and the bottom one shows 15. For species that we might find around here in a cultivated environment, The Sibley Guide to Trees lists the following numbers of leaflets:

Black walnut: 15-23

Butternut: 11-17

Japanese walnut: 11-17

English walnut: 7-9

Shagbark hickory: 5-7

Pecan: 11-17

Bitternut hickory: 7-11

So, based on the number of leaflets, it could be butternut, Japanese walnut, or pecan.

What about the nut? In the Juglandaceae section of The Sibley Guide to Trees, the following nuts seem to match the ones on the tree that I see: black walnut, English walnut, and shagbark hickory.  

I don't think it could be shagbark hickory because it does not have enough leaflets.  The butternut, Japanese walnut, and pecan nuts just aren't round enough.  

The book says that English walnut has toothless leaves.  Clearly these leaves are toothed.  So I'm thinking maybe it's a black walnut.  Maybe the leaf with 11 leaflets was just a fluke.  Also, the book says black walnut is large, which fits -- this tree is pretty tall.   I'll have to go back and check out some more leaves.