Leviticus 11:3

Hoof divided, and cheweth the cud. The dividing the hoof, and chewing the cud, signify discretion between good and evil, and meditating on the law of God: and where either of these is wanting, a man is unclean. In like manner, fishes were reputed unclean that had not fins and scales: that is, souls that did not raise themselves up by prayer, and cover themselves with the scales of virtues, (Challoner) particularly of mortification and penance. (Worthington)