James Gairdner and John Horace Round, 1911 Encyclopædia Britannica, Volume 21… (1911)
“ The representation of the earlier Percys had passed away through the daughters of Earl Thomas, beheaded in 1572, but his earldom of Northumberland (created anew for him in 1557) had passed to his brother Henry, under a special remainder, and appears to have become extinct in 1670, though persistently claimed by James Percy, “the trunk-maker.” The last earl's daughter Elizabeth, a great heiress, was mother by Charles Seymour, 6th duke of Somerset, of Algernon, 7th duke, who was summoned (in error) as Lord Percy in 1722 and created earl of Northumberland in 1749. ”
