Mary Delafontaine (The Pale Horse)

In the novel The Pale Horse, Mrs Mary Delafontaine was a friend of Ariadne Oliver. When talking to Mark Easterbrook, Mark described a fight between two girls which he had witnessed in a café. Thomasina Tuckerton, one of the girls, seemed to have her hair come off easily, pull out at the roots without much pain by the other girl.. Ariadne then recalled how she had visited her friend Mary Delafontaine in a nursing home and saw that her hair was falling off.

Later in the book, Mary Delafontaine died, with the cause of death diagnosed as "toxic polyneuritis". Her money was left to a cousin who at the time of her death was in Kenya with her husband.

Like Thomasina Tuckerton, the name Delafontaine was present on a list of names found in the shoe of the murdered Father Gorman. When Mark Easterbrook learned later that Lady Hesketh-Dubois, his godmother, whose name was also on the list and she had also lost a lot of hair just before dying, he connected it to a scientific article he had read recently and surmised that, although they all seemed to have died of natural causes, they had actually all been poisoned with thallium.

Despite the relatively uncommon name, "Mary Delafontaine" actually occurs at least twice as different characters in Christie's works.