Chan and Zen count Mahakasyapa as the first patriarch of a transmission that runs outside the scriptures, and the story they tell about it is very short. The Buddha, before an assembly waiting for a discourse, held up a flower and said nothing. Nobody understood except Mahakasyapa, who smiled. The Buddha then said that he possessed a treasury of the true dharma eye, a special transmission outside the teachings, not established on words and letters, and that he entrusted it to Mahakasyapa.
The story does not appear in the Indian canon. It surfaces in Chinese texts of the Song period, more than a millennium after the events it describes, in a tradition that needed an unbroken chain of mind-to-mind transmission running back to the Buddha in order to authorise its own teachers. The atlas records it as what it is: a Chan foundation narrative of demonstrable date, and one that has done an enormous amount of work.
The choice of Mahakasyapa is not arbitrary. He was already the figure who took charge when the founder died and fixed what would be transmitted, so a school claiming to hold something the texts did not contain had an obvious candidate. He also has a long afterlife in another direction: a widespread tradition holds that he did not die but entered a state of suspension inside a mountain, holding the Buddha's robe until the next Buddha comes to receive it.