Both lions and leopards may have been among the earliest beasts to appear in heraldry. "The Oxford Guide to Heraldry" notes that the earliest English treatise on heraldry, a late-13th or early-14th century Anglo-Norman manuscript titled "De Heraudrie", mentions the crow, eagle, griffin, heron, leopard, lion, martlet, popinjay, and swan. Citing Bado Aureo, the "Oxford Guide" further suggests that the leopard, said to be "borne of an adulterous union between a lioness and a pard," and like a mule incapable of reproducing, may be an appropriate charge for a person born of adultery or barred from reproducing (such as an abbot).