Note 035
From Chapter 20 of the Decline & Fall

Transversa X literâ, summo capite circumflexo, Christum in scutis notat. Cacilius de M. P. c. 44.

Cuper (ad M. P. in edit. Lactant. tom. ii. p. 500) and Baronius (A.D. 312, No. 25) have engraved from ancient monuments several specimens - as thus,
or of these monograms, which became extremely fashionable in the Christian world.