Hooded Keyhole Limpet (Cranopsis cucullata, formerly Puncturella cucullata)
Photographed on 12 March 2016. This species is more common subtidally — I've only seen a handful washed up on the beach in this area.
The "keyhole" is a normal feature, i.e., this is *not* a broken shell. The hole, or slit, is located just in front of the apex (peak) of the shell. The position and shape of the keyhole, along with the strong ribs and the dentate margin, are useful identification features.
P.S. The species name, "cucullata," means "hooded" — sometimes the apex of the shell leans backward (like a hood). [I should have taken a picture from the side; I'll try to add one.] This expands the list of "hooded species" with cucullata in their scientific name — see the post from 18 June 2015.