Basis for ID is that it looks rounder than oblong (Tresus nuttalli)
By boat launch. Very likely dropped on the beach from a boat.
I think.
I found it upside down and the it righted itself.
Was found growing just north of the Golden Garden beaver pond in the mid-intertidal zone. Was associated with other species of seaweed such as ulva, and was in moderate abundance. Was growing attached to a rock, and was in the sporophyte phase of its life cycle.