I remember a schooltrip we made to a museum. Some old guy told us an anecdote about mushroom books. How was it determined how many skulls a mushroom should get?

I remember a schooltrip we made to a museum. Some old guy told us an anecdote about mushroom books. How was it determined how many skulls a mushroom should get? Well, he explained there was this guy who had eaten poisonous mushrooms for the purposes of rating them. One mushroom he had eaten made him very sick and he was all alone, and apparently barely made it out alive. That one got three skulls. Buyers beware, and if you are not, thank you for your sacrifice.