Fraser
Jean's advice to search the 1891 census to verify Susan's birth place is good. . I've double checked the IGI using just the batch code for Dundee, C112821 (which gives more flexibility on the search parameters) and nothing.
I have also looked at the 1881 Census and there are two possibilities:
1. There is a Susan Arthur, age 13, b. England, showing as neice to David and Sarah Bissett in Arthurstone Mains, Coupar Angus, Angus.
2. There is one more entry for a Susan McArthur, age 18, b. Dundee living with her father William McArthur, widower, Coffee House Keeper also born in Dundee. Living with them are children Ellen, age 16, and Alexander, age 13. Address is Waverley Buildings, Liff and Benvie, Angus. This is the only entry for a Susan A*thur* born in Dundee, which also picked up on the Mc spelling on the 1881 census.
This is the IGI entry for that Susan's birth, as you can see, the mother's details don't match:
SUSAN MC ARTHUR Birth: 14 OCT 1861 Dundee, Angus, Scotland
Parents: Father: WILLIAM MC ARTHUR Mother: MARGARET RAMSAY
My final check was for the death of a Mary Dunn other name (M*c)A*thur* anytime between 1855-1950. There are only two and both have wrong birth years (1855 and 1894).
A bit of a puzzle this one! Hopefully the marriage cert in 1890 might give more clues (were her parents showing as deceased for example, occupation of Susan's father, names of witnesses). Also the birthplace showing on the 1891 census. Finally, I would look at the name and order of names for Susan and Alexander's children to see whether they following Scottish naming patterns.
Regards.
Monica