Complex sentence: The account states that this Olaus was the reputed ancestor of the MacLewis, or Fullarton family, which originated on the Isle of Arran; and that this family traced its ancestry from Lewis, or MacLoy, son of Olaus, son of Leod.
Split into simpler sentences: The account states that this Olaus was the reputed ancestor of the MacLewis, or Fullarton family, which originated on the Isle of Arran. This family traced its ancestry from Lewis, or MacLoy, son of Olaus, son of Leod.