They are more or less angular, and while elongated transversely at first, become nearly Isodiametric as the pileus becomes fully expanded, passing from an elongated form to rectangular, or sinuous in outline, the margin more or less upturned, especially in age, when they begin to loosen and "peel" from the surface of the cap.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc."
George Francis Atkinson