Usually occuring in older members of society, "Tombstone teeth" is a derogatory term used to describe a mouth consisting mainly of tongue and gums. Maybe, one or two teeth will stand out but no more than that. The ones on the bottom jaw seem more resistent to base-jumping out of the mouth, for some reason. Scientists are looking into this as I speak.
"That old Mrs Webster has some serious tombstone teeth. They're all yellow..."