Black curves show the modeled evolution of the radii for the first ten shells in the gravitational potential of NGC 4993. The gray transparent regions correspond to the uncertainties in the halo mass and concentration. Red (blue) lines correspond to the actual shell edges north (south) of the center of NGC 4993. The transparent red and blue stripes correspond to the measurement errors of the shell radii, ±5%, measured on the major axis of the galaxy.