Ohalot, Chapter Fifteen, Mishnah Nine
1) A jar full of clean liquid and sealed with a tightly fitting lid, which had been made the covering stone of a tomb, a person touching it contracts seven-day uncleanness but the jar and the liquid remain clean.
2) An animal that had been used as a covering stone, a person touching it contracts seven-day uncleanness.
3) Rabbi Meir says: anything possessing the breath of life does convey uncleanness on account of [its being used as] a covering stone.
Section one: The jar used to cover the tomb conveys impurity to one who touches it, just as does a stone. However, an earthenware jar is not defiled on its outside, as long as it is covered with a tightly fitting lid (see Kelim 9:2). Therefore, the jar and its contents remain pure.
Section two: Similarly, if a person uses an animal to temporarily cover a tomb, it too conveys impurity. Note that like the jar, the animal is not impure. It merely conveys impurity to one who touches it. See also Eruvin 1:7.
Section three: Rabbi Meir says that anything that is alive does not convey uncleanness if used to cover a tomb. This would mean that he disagrees with the halakhah in section two, although not with the halakhah in section one.