Niddah, Chapter 6, Mishnah 4. As this chapter draws toward its close, the Mishnah presents a series of paired rulings, each one built on the same pattern: whatever qualifies for one halachah automatically qualifies for a second, while the reverse is not always true. Our mishnah applies that pattern to two kinds of tumah that an object can receive.
The Mishnah states: "Kol hamitamei midras, mitamei tmei meis." Anything that is capable of becoming tamei through midras, that is, through a tamei person treading or resting his weight upon it, is likewise capable of becoming tamei with the tumah that comes from contact with a dead body, tumas meis.
The Mishnah then adds the second half of the pair: "V'yeish mitamei tmei meis v'eino mitamei midras." There exist objects that do become tamei through contact with a dead person, and yet those same objects are not subject to the tumah of midras at all.
The relationship, then, runs in one direction only. Susceptibility to midras carries susceptibility to tumas meis along with it, so that an item included in the first category is automatically included in the second. But an item may well belong to the category of things that receive tumas meis without belonging to the category of things that receive midras.
This is the shape of the mishnah: one list is contained entirely within the other, and the wider list of tumas meis reaches items that the narrower list of midras does not.