Looks a lot like Pluerotus ostreatus -- the Oyster mushroom. According to Arora (Mushrooms Demystified -- a great guide, and he is located in CA, so there is a focus in that area).
This fungus causes decay in live trees as well as inhabits logs and snags. usually found on cottonwoods, elm. sycamore, tanoak, and elm. i suppose Eucs aren't ruled out.
Did you ever do that dissection of the trunk?
In the future, if you want to collect info ion a fungus, describe it in its entirety: cap color and feel when wet and dry; gills, crowded, sparse, forked, on stem or just cap; spore color (take a spore print by putting a cap under a glass for a few hrs), inside and outside of the stem, overall dimensions, habit (clustered or single), mycelia (color, presence as fans under bark or thick layers in wood), color of incipient decay (wood more or less hard but a different color), smell, presence/absence of zone lines in the wood, and substrate (live tree, deadwood, log, ground). Plus, take one and leave it in a paper bag to dry; an expert can tell a lot from a dried specimen 9especially if you include the fresh specimen notes).
Everything about this 'shroom says oyster mushroom, at least based on what is in the photo.
opcorn:
