Mishnà su Kelim 8:5

A dog which had eaten the flesh of a corpse and then died and was lying over the threshold:Rabbi Meir says: if its neck has a thickness of one handbreadth it can bring the uncleanness, but if not, it does not bring the uncleanness. Rabbi Yose says: we [examine to] see where the uncleanness is. If it is beneath the lintel and inwards, the house becomes unclean; if from the lintel and outwards, the house remains clean. Rabbi Eliezer says: if its mouth [points] inwards, the house remains clean; if its mouth [points] outwards, the house becomes unclean, since the uncleanness goes out through its hind. Rabbi Judah ben Batera says: in either case the house becomes unclean. How long can [the uncleanness] remain in its entrails? Three whole days. [If in the entrails] of fishes or birds, as long as [it takes for the uncleanness] to fall in the fire and be consumed, the words of Rabbi Shimon. Rabbi Judah ben Batera says: in the case of fishes or birds, twenty-four hours.