Crux (krŭks), n.; pl. E. Cruxes (-&ebreve_;z), L. Cruces (kr&usuml_;sēz). [L., cross, torture, trouble.] Anything that is very puzzling or difficult to explain. Dr. Sheridan.
[1913 Webster]

The perpetual crux of New Testament chronologists. Strauss.
[1913 Webster]