Faint disclination lines which vanish at the transition to the uniaxial nematic phase, are observed in the biaxial nematic phase of a ternary lyotropic mixture. Unlike the m-disclination lines, they do not exhibit observable breaks. They bear ghostlike point defects instead. The joint observation of both kinds of disclination lines could give a real proof of biaxiality, as well as the evidence of zigzagging disclination lines with temperature dependent breaking angles. Conversely, a quick analysis of the disclination lines observed in a uniaxial nematic phase may erroneously lead one to conclude to biaxiality.