The Old Men's cross is most often called a 'quincunx'. The word is attractive to Joyce because it can readily be made to suggest both a sexual vulgarism and the 'Tunc' page of the Book of Kells (folio 124r) - This page, bearing part of the text of the Crucifixion - Tunc crucifixerunt XPI cum eo duos latrones - is in fact illustrated in the lower half by a large crux decussata. The allusions to it which are heard again and again throughout Finnegans Wake are always associated with the cross of the Four and with the female organ. The clearest association of 'quincunx', 'Tunc', etc., is at 278.L1 'Pitchcap and triangle, noose and tinctunc'. The last word here is plainly 'quincunx', and thus the four symbols named are those for Shaun (^), ALP (triangle), the 'Twelve' (O), and the 'Four' (x). Mr. Atherton has many interesting things to say about Joyce's conception of Original Sin as God's sexual fall entailed in the act of Creating.' The further implication in his use of 'Tunc' seems to be that Christ's 'fall' on the cross, far from redeeming Adam's sexual fall, is in fact to be identified with it and with the fall of God the Father. Joyce was always contemptuous of Christ's virginity: 'He was a bachelor . . . and never lived with a woman. Surely living with a woman is one of the most difficult things a man has to do, and he never did it'. 

Hart, Clive / Structure and motif in Finnegans wake