Late 1880's Sidesaddle Habit
This photo dates from about 1887. It shows how the apron has started to become more managible, and how the bodice is evolving from being based on a traditional bodice to becoming a jacket. Although the bodice is becoming more jacket like, they will still remain quite quite fitting until the 1900's. Note how the skirt looks to have a bustle at the back, when not mounted. Late 19th century aprons could be draped across your side and fasten at the hip so they wouldn't drag on the floor (they are still fastened in this fashion even today with the tailored skirts), however,
I have a feeling that she is wearing a bustle underneath the apron for fashion's sake. She probaly didn't wear her habit to the photographers but got changed into it there and kept her bustle on (bustles or hoops were not worn while riding). This is not uncommon. I once saw an 1860's CDV of a woman in a riding habit with her hoops still under it!