While walking along the Mokelumne River earlier today, we came upon these interesting nuts/seeds? The trees bearing these appeared to be already dormant as there were no leaves on the tree. It did not appear that the deer, squirrels or birds had found these worthy of their efforts. Was thinking some sort of wild chestnut? I took the husk off and cut into it with my knife in the photo below.

Looks like Buck eye, especially Aesculus californica fruit

California Buckeye (Aesculus californica). I am new here, so I am not sure about citing sources, but from Wikipedia: Aesculus californica, commonly known as the California buckeye or California horse-chestnut, is a species of buckeye native to California and southwestern Oregon. Thank you tofitropic!