The Settlers of the Beekman Patent, Dutchess County, New York by Frank J. Doherty, 2:357-358, states that Thankful BANGS, widow of Samuel BANGS, was married after Samuel's death to "Isaac Crosby 11 Nov. 1799 at South East. They were both reported by the newspaper to be over 80 years of age. Thankful Crosby, wife of Isaac d. 19 Feb. 1815 in 87th year. Isaac Crosby died 17 Feb. 1815 in 97thyear. [MDDC 3, OGPC 329]." See http://www.newenglandancestors.org/research/database/beekman/default.asp?file=RESEARCH\DATABASE\BEEKMAN\CONTENT\VOL.02.358.HTM
MDDC = Notices of Marriages and Deaths Published in Newspapers printed at Poughkeepsie, New York 1778-1825 in Volume IV of the Collections of the Dutchess County Historical Society
OGPC = Old Gravestones of Putnam Co. NY. By BarbaraSmith Buys, Gateway Press 1975
If Isaac really had 19 children, he probably had at least one other wife, whose name is unknown, between Mercy FOSTER and his marriage to Mrs. Thankful BANGS in 1799.