|SOME NOTES ON MEDIEVAL ENGLISH GENEALOGY|
Volume 5, pages 357, 358 (as modified by volume 14):
He [Sir Henry Ferrers, son of William (de Ferrers), Lord Ferrers of Groby (d. 1445)] m., shortly before 13 July 1416,(b) Isabel, 1st da. and in her issue [heir] of Thomas (MOUBRAY), DUKE OF NORFOLK AND EARL MARSHAL, by his 2nd wife, Elizabeth, sister and coh. of Thomas, 12th or 5th EARL OF ARUNDEL, and 1st da. of Richard, 11th or 4th EARL OF ARUNDEL.
Note b (continuation on page 358):
This Isabel was, in her issue, the senior coh. to Anne, da. and h. of John, Duke of Norfolk, grandson of John, Duke of Norfolk, br. of the said Isabel.
Volume 9, page 610, note c, suggests that Isabel may have been younger than her sister Margaret, since John Howard, Margaret's son, was later created Duke of Norfolk.