how would you describe the Struldbruggs?

From the text:

"One Day in much good Company I was asked by a Person of Quality, whether I had seen any of their Struldbruggs, or Immortals. I said I had not, and desired he would explain to me what he meant by such an Appellation applied to a mortal Creature. He told me, that sometimes, though very rarely, a Child happened to be born in a Family with a red circular Spot in the Forehead, directly over the left Eyebrow, which was an infallible Mark that it should never dye. The Spot, as he described it, was about the Compass of a Silver Threepence, but in the Course of Time grew larger, and changed its Colour; for at twelve Years old it became Green, so continued till Five and Twenty, then turned to a deep Blue; at Five and Forty it grew coal Black, and as large as an English Shilling, but never admitted any further Alteration. He said these Births were so rare, that he did not believe there could be above Eleven Hundred Struldbruggs of both Sexes in the whole Kingdom, of which he computed about Fifty in the Metropolis, and among the rest a young Girl born about three Years ago. That these Productions were not peculiar to any Family, but a meer Effect of Chance; and the Children of the Struldbruggs themselves, were equally Mortal with the rest of the People."