Tricholoma mutabile
Tricholoma mutabile is a mushroom of the agaric genus Tricholoma. Found in Yuba County, California, it was first described scientifically in 1996. It has a grayish convex cap that is  wide, a white stalk measuring  long by  thick. The white gills are sinuate, and turn pale golden brown in maturity. See also
List of North American Tricholoma
List of Tricholoma species

References

mutabile
Fungi described in 1996
Fungi of North America