Boy...this is a REAL mystery!Usually I'm able to dig something up...but this time, not a snippett.
Have you written off for the marriage certificate for the 1900 census, James Hart's daughter Tillie?This might give you some information regarding the maiden name of her mother and confirm or deny what you believe at this time.
I viewed the 1900 census and with Thomas only a few doors down, it would seem that this is YOUR James.Also, James and his wife claim that they have been married for 37 years.That would put their marriage date around 1863 and I believe that would conflict with your Betsy Cady marriage.Also, IF the James Hart listed in the 1900 census IS related to the Thomas Hart that is there, then James daughter Tillie would have been born in 1877.There is NO female listed in Hart/Cady household in the 1880 census where James lists himself as a widower.However, if we are NOT to believe that James and Clar? have actually been married 37 years, and that this a second marriage for them, then it is certainly possible that Tillie is Clar?'s daughter.
Regardless of where they've been for the other census', you may still want to order a copy of Tillie's marriage certificate.Notice that they've been married 0 years.There is a real good chance that they were married in Waushara County.You can try this link to the Wisconsin Historical Society:
The price for a record search is reasonable and their service is fairly quick.Since you know the name of the bride, location and approximate date of marriage, they should be able to find a record for you.
I hope this has been helpful.I wish you continued good luck with your research.