Merle Haughwout left her surname blank in his NYGBR article; in his subsequent chart, he specified Titus without any further explanation. If his information comprised more than just a guess based on the name of her third son, then the identity that I propose here seems to be the likely and logical solution, but must still be considered conjecture until more direct proof is found.
The Titus family has been extensively researched, and for years no known Mary Titus could be identified with her. In his 2004 book, Leo Titus added a Mary to the previously known children of John Titus and Sarah Platt, the most likely local Titus family to have a connection, especially as two of Harmon and Mary's children (Adam and Abigail) wed two of John's grandchildren (Rebecca Conklin and Henry Titus, respectively).
It appears that he did this based on a reference in John Titus' will (dated April 22, 1754) to a daughter "Mary Hugens", supposing this to be a married name. However, she was surely an unmarried stepdaughter, her mother being Martha Oldfield, widow of James Huggins, who married John Titus on February 21, 1743.
Mary Hugens is bequeathed one bed, and is mentioned right after John Titus' daughter Elizabeth Titus, who receives the same bequest. Elizabeth was about sixteen and unmarried at the time; all things considered, we may infer that Mary was of a similar age.
In the baptismal records of St. George's Church, Hempstead, NY, there are five children of James Huggins and Martha Oldfield attested: Sarah (Nov. 11, 1734), Milissent, James, and another Sarah (all Aug. 7, 1738), and Porochis (a son, Sept. 27, 1738 -- both the name and the timing are highly unusual). Mary was no doubt born no earlier than 1739, and very close to her father's death, which has been placed at 1741. (This may be just a guess based on the date of Martha's remarriage, but the possible range isn't large anyway.)
This would serve to explain why Mrs. Mary Lefferts, wife of Harmon, was baptized at St. George's as an adult along with her first two young sons, Adam and James, on Sep. 9, 1759. It would also possibly explain why her second son was named James, with no James being in either the recent Lefferts or putative Titus background. (The name of the first son, Adam, remains mysterious, though; it may point to someone farther back on the line of James Huggins, about whom not much seems to be known. The name does not occur anywhere else in the Lefferts family, or in the Titus family of the time, but tends to show up in the prominent Quaker families of the area.)
The timing of events in general would also make much sense. John Titus died on June 4, 1754, leaving young Mary without a stepfather, and giving her every incentive to marry at sixteen (or even possibly slightly younger than that; at any rate, her mother had first married at sixteen). Harmon and Mary's first child, Adam, is born on January 6, 1756.
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