Note the progression from the "button" stage to globose and then to broad and plate-like. Apparently some can reach up to 40 cm (~15 inches) across!
The small, flattened patches on the cap are remnants of the universal veil — a layer of tissue that initially covers the emerging mushroom.
While scanning different mushrooms among the pine needles, we noticed that a few had been nibbled:
After a little more searching, we discovered the likely culprit. Can you find it in the image below?
Yes! A banana slug (Ariolimax sp.) can be seen near the base of the mushroom (see close-up below).
(Athough slugs eat them, beware that Fly Agarics are poisonous to people.)
Peter recalled that occasionally these distinctive red-and-white mushrooms show up on winter holiday cards. This U.S. Forest Service ethnobotany page discusses this association and presents a few images of vintage cards with examples. I especially like the one with children collecting mushrooms in the snow — a good idea for a knitted hat!