"'Step across this,' [Math] said, 'and if you are a maiden, I will know it.'
"She stepped across the rod then, and as she did the dropped a big, fine, yellow-haired boy. What the boy did was to utter a loud cry. After the boy's cry, she made for the door, dropping as she went some little thing from her. Before anyone could have a second glance at it, Gwydion took it, wrapped a brocaded silk coverlet around it and hid it. Where he hid it was in a small chest at the foot of his bed.
"'Well,' said Math son of Mathonwy, 'I will have this one baptized,' he said, looking at the sturdy yellow-haired boy. 'The name I will give him is Dylan."
Dylan, interestingly enough, is a Welsh word meaning "wave."
"The boy was baptized, and as soon as he was, he made for the sea. No sooner had he come to the sea than he took the sea's nature, and swam as well as the best fish in the sea; because of that he was called Dylan Eil Ton. No wave ever broke under him..."
Patrick Ford writes of the scene, "In the fourth branch of the Mabinogi, Math induces birth in Arianrhod. She delivers twins, one of whom is christened Dylan; he immediately makes for the sea and assumes its nature. One is tempted to believe that whoever sired this child also came from the sea, although the principles of genetics do not always dictate the shape of the mythic narrative. The other of this pair is Lleu Llaw Gyffes, whose name is cognate with that of the Irish Lugh, foster-son of Mannannan Mac Lir, the Irish sea god."
Dylan is mentioned few other times in Celtic mythology. As a baby, Taliesin (who is considered to be synonymous with the later myths of Merddyn, or Merlin) is said to have sung, "Though I am frail and little/And wet with the spume of Dylan's sea." It is told that Dylan was killed by his uncle, Gofannon, and that this was one of the "three unfortunate blows" in Celtic lore.
Dylan's story doesn't seem really related to the stories of selkies in its own right, but it is incoporated into Rosalie Fry's "The Secret of Roan Inish," in which a young boy -- who is the descendant of a selkie -- takes to the sea at birth and rides on the waves in his cradle boat.
Charles Squire writes, "Of this connection [between Math and Arianrhod] two sons were born at one birth -- Dylan and Lleu, who are considered as representing the twin powers of darkness and light. With darkness the sea was inseparably connected by the Celts, and, as soon as the dark twin was born and named, he plunged headlong into his native element. 'And immediately when he was in the sea,' says the Mabinogi... 'he took its nature, and swam as well as the best fish that was therein. And for that reason, he was called Dylan, the Son of the Wave. Beneath him no wave ever broke.
"He was killed with a spear at last by his uncle, Govannan, and, according to the bard Taliesin, the waves of Britain, Ireland, Scotland and the Isle of Man wept for him. Beautiful legends grew up around his death. The clamour of the waves dashing upon the beach is their expression of their longing to avenge their son. The sound of the sea rushing up the mouth of the River Conway is still known as 'Dylan's Death-Groan.' A small promontory on the Carnarvonshire side of the Menai Straits, called Pwynt Maen Tylen, or Pwynt Maen Dulan, preserves his name."
Squire, as many Celtic mythologists, argues that a number of mythological figures eventually turn up in the ultimate Celtic tale, that of King Arthur. He writes, "Gwydion was the father ... of twin sons, Lleu, a god of light, and Dylan, a god of darkness; and we find the same story woven into the very innermost texture of the legend of Arthur. The new Arianrhod, though called 'Morgawse" by Sir Thomas Malory, and 'Anna' by Geoffrey of Monmouth, is known to earlier Welsh myth as 'Gwyar' ... and the new Dylan is Medrawt, at once Arthur's son and Gwalchmei's brother, and the bitterest enemy of both." Instead of Dylan taking peacefully to the sea, and his death mourned, Medrawt -- Mordred -- betrays his father and becomes an enemy of the kingdom. The 'dark side' of this mythological figure is what seems to rise to the top.