The 1860 census for Sheboygan Cty, town of Lima, page 487, taken 5 July 1860, shows a Darius Dart, age 30, Farm Laborer, $75 personal worth, born in NY.His wife was Sarah M., age 18, born NY.They had a son, Almand, 9 mos old, born in WI.This was the only Dart listed in Sheboygan Cty. in 1850 or 1860.From what you listed in the post, this sounds like your Dart.
Found one Dart in the Old Cemetery index in the Mead Public Library.This could be Darius's mother, maybe? Hingham Cemetery, Town of Lima, Sheboygan County Mrs. Dart, died Feb 19, 1857, aged 68 years.
Dart is an uncommon name in Sheboygan County.Checked the marriage index on the Sheboygan GenWeb site and there wasn't a Dart listed, though marriage records can be a little spotty during that time.