Complete Peerage - Marshal, Vol XIV, p. 468 la donne par erreur comme fille de Robert III de Ferrers.
In summary, evidence has been presented that Elizabeth de Ferrers, wifeof Sir William le Marshal, and Sir David ap Gruffudd, was the daughterof William de Ferrers, 5th Earl of Derby, by his 2nd wife, Margaret (or Margery) de Quincy. It has also been shown that before his marriageto Elizabeth de Ferrers, Sir William le Marshal was previously marriedto Pernel del Ortiay (or de Lorty), by whom he appears to have had twosons, John and William. By her 2nd marriage to Sir David ap Gruffudd,Elizabeth de Ferrers is thought to have had two sons, Llywelyn and Owain, and seven daughters, including Gwladus. It is thought that all ninechildren died young or without issue, so Elizabeth de Ferrers has no known living descendants.(Douglas Richardson)