Hi Ralph, John Jr. and Jenny Ferguson COULD have had two daughters named Mary. It was not uncommon when a child died to name the next or subsequent child with the same name. It happened in my family in Illinois 1850-1860.
The only way to know for sure is to find additonal on John Jr. and Jenny. You won't find it in the Censuses because 1840 and prior they only listed the head of household.
You might find the info in their obituaries. Successful Searching!