The way I remember it [hazily] there are verses for every number from two to ten:

The animals came in two by two
(There's one more river to cross)
The elephant and the kangaroo
(One more river to cross)

Refrain

The animals came in eight by eight
"Hurry up!" says Noah, "Or I'll shut the gate"

Hope this helps a bit...I'll pick my brother's brain for more...