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Male Seth Loomis Click to view Seth Loomis in the family tree

Seth was born on May 22nd, 1737 in Westfield, MA.1   Seth's father was Joshua Loomis and his mother was Abigail Langdon.  His paternal grandparents were William Loomis and Martha Morley; his maternal grandparents were John Langdon and Abigail Phelps. He had five brothers and five sisters, named John, Joshua, Enoch, Justus, Justus, Abigail, Lois, Abigail, Ann and Martha.2 

1 phelps.FTW @ Tree #0105
2 phelps.FTW @ Tree #1353
Further sources/citations:
World Family Tree Vol. 3, Ed. 1 @ Tree #3669, 3669.ftw, World Family Tree Vol. 4, Ed. 1 @ Tree #0574, 0574.ftw