Clarissa Brown born Elizabethtown, Leeds County, Ontario, Canada d/o Ezra Brown FAG128829116 and Elizabeth Newell. Ancestry.com had her name as Rebecca Clarissa Brown. Her name is
Clarissa Brown. I believe an error on one person entry in Ancestry.com/Public Members and everyone then followed suit. All documentation I have found and tombstone is Clarissa Brown McMillan.
Confusion with Morrison McMillan's 2nd wife
Rebecca Childs Chapman who he married 14 June 1858
Lacrosse, Lacrosse Co., WI.

1860 Sparta, Monroe Co., Wisconsin
R. C. McMillan 52 500 200 b. Mass.(Rebecca Childs)
R. S. Chapman 23 b. Conn (son from 1st m.)
Mary McMillan 17 b. Ohio
R. C. Crockston 24 Farmer b. New York
(Mary Arnott McMillan is the 12th child of Morrison McMillan and 1st wife Clarissa Brown who married a Edward E. Alcorn)

Clarris Brown married Morrison McMillan s/o John McMillan and Rachel Morrison 15 Feb. 1821 Huron Co., Ohio. Book "McMillan Genealogy and History" by W.F.Mcmillan and C.E. McMillan (1908) pg. 95-96
states married 1820 Seneca Co., OH.
She had 13 children:
1. Rachel McMillan died small child
2. John McMillan m. Elizabeth Carmichael
3. Orlando McMillan m. Miss Hayden
4. Charlotte McMillan m. Wooster Fay
5. Agnes Jane McMillan m. Resalvo S. Kingman
6. Matilda Cherilla McMillan m. Emory Sprague Lathrop
7. Morrison McMillan Jr. m. Sarah Stowe, Lucinda Mellot
8. Sarah B. McMillan m. Barrett E. Blodgett
9. Daniel McMillan d. small child
10. child McMillan d. infancy 1837
11. child McMillan d. infancy 1837
12. Mary Arnott McMillan m. Edward E. Alcorn
13. Amanda McMillan d. infancy

Above McMillan Genealogy and History by W.F. McMillan and C. E. McMillan 1908 Book Reference: obituary
published in "Monroe County Freeman"
April 16, 1857
"Clarissa McMillan was oldest daughter of Ezra Brown and wife, formerly of Kinderhook, Columbia Co., N.Y. He resided in Canada for some years, where he accumulated a handsome property; which he gave up and returned to the United States in 1813, rather than become a British subject. Clarissa was brought up by a devoted Christian mother and ever ordered her life by those early teachings. She was a model wife and mother, a kind neighbor and friend, and her death is mourned by a large circle of acquaintances. Appropriate resolutions were adopted by the Masonic lodge of Sparta, Wis., extending sympathy and condolence to Brother Morrison McMillan in his bereavement."

Monroe County Museum and Library does not have this newspaper on microfilm it was taken from above book reference.

Her sister Achsah Brown m. Benjamin Sill McMillan a 1st cousin of Morrison McMillan.

Pope-Brown-Newell-McMillan-Lathrop Genealogy
Diane L. Webb
FAG47297921
Relationship: Clarissa Brown McMillian was sister to my
2nd great grandmother Nancy Jane Brown Pope wife married to Cyrus Waterman Pope of Sparta, Monroe Co., WI.