This is a picture from the French magazine Journal des Demoiselles, from January 1877, showing the back of two ladies in evening-dresses - and a young girl, also with her back turned to the spectator.
The 1880's had its bustle era, as can be seen here, and so did the 1870's, but there were a few years in between when the bustle was gone - or at least almost so. These dresses are from this period and instead the skirts are very slim and narrow (but note that the bustle is not completely gone from the little girl's dress). Whether there was still a bustle or not on the dress in 1877 was different from gown to gown - a matter of taste and how daring the wearer chose to be. And it is also worth remembering that fashion seldom is very clear-cut, with one day the dresses looking one way and the next day completely different. Instead it is a gradual change.
Some things that really did not change was how to choose accessories for the evening-dress - like it had been twenty years earlier and would be twenty years later there were the long gloves and the fans. The hair and the dresses were adorned with artificial flowers and the dresses of a cut that would mimic the current fashion - and take everything one step further, the trains being longer, the neckline being lower (just like evening-wear is today - though trains are not that common).
What is more enigmatic is the black-dressed girl. It almost looks like she is in mourning - why would she otherwise be dressed in black? A girl in black seems like a strange companion for women in party-clothes - but mixing different kinds of dresses on these fashion plates was quite common so perhaps it is not that surprising, even the subjects seem to be an ill match.