This image allows this and this image to be dated.
From spanishroyals.tumblr.com/page/110; spots remove throughout image with Photoshop.
This would be a good photo to see in color. Maria's dress appears to have two lapels; just where her innermost neckline is cannot be resolved.
Keywords: 1877, Queen Maria de las Mercedes, Princess, Queen, Bourbon family, Borbón family, French, straight coiffure, jacket, vee neckline, lapels, bows, buttons, long close sleeves, cuffs, ruffles, cuirasse bodice, bustle, close skirt, ruching, train, lining, necklace, earrings, fan