According to CP XI:368 note (a), William inherited after hiselder brothers Herbert & John died without issue, and his nexteldest brother Anselm's two sons, William and Hugh died withoutissue. William then inherited from his nephew.
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William St. Quintin had, by Beatrix, his wife, a son, Herbert,Baron of St. Quintin. [Sir Bernard Burke, Dormant, Abeyant,Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London,1883, p. 469, St. Quintin, Baron St. Quintin]
Sir William, lord of St. Quintin, m. Beatrix, daughter of Saire,Lord Sutton, of Holderness, and from him descended the BaronsSt. Quintin. [John Burke & John Bernard Burke, Extinct andDormant Baronetcies of England, Ireland, and Scotland, SecondEdition, Scott, Webster, & Geary, London, 1841, p. 463, St.Quintin, of Harpham]