A Sheretz the size of a lentil conveys Tum'ah.
It is a Safek whether a person who eats a Sheretz that is lacking a leg is punished with Malkus. (1)
If most of the ribs from a spine of a Mes are broken it is Tahor. (2)
There was a Machlokes between R. Akiva and the Chachamim regarding a limb from a Chai or a Mes, a Revi'is of blood and a half of a Kav of bones that come from two Mesim. (3)
R. Akiva originally held that a bone the size of a barley that was split in two is Metamei, but R. Yochanan Ben Nuri argued.
R. Akiva originally held that a spine or skull that come from two Mesim is Metamei according to R. Akiva, but the Chachamim are Metaher.
A bone the size of a barley is not Metamei b'Ohel. (4)
A Rova Kav of bones that come from Rov Minyan or Rov Binyan is Metamei b'Ohel according to Beis Hillel.
Shamai says that a bone from the skull or spine of a Mes is Metamei b'Ohel.
Rami Bar Chama asks whether a Nazir shaves for the Tum'ah of a Rova Kav of bones from the skull or spine of a Mes. (5)
A BIT MORE
1. If a person eats a complete Sheretz he is punished with Malkus even if it is less than a k'Zayis.
2. If it is in a grave it conveys Tum'ah because the grave combines it.
3. R. Akiva was Metamei and the Chachamim are Metaher. R. Akiva retracted his position regarding all of them other than a Revi'is of blood that come from two Mesim.
4. It only conveys Tum'ah via touching and carrying.
5. Even though the Mishnah says that a Nazir only shaves for a half of a Kav of bones, if the bones are from the skull or spine maybe even a Rova Kav is Metamei.
THE TUM'AH OF THE SPINE AND SKULL
The Rambam rules that a spine or skull of a Mes is Metamei. The Kesef Mishneh asks that it is an unresolved Safek to the Gemara whether the skull or spine are Metamei separately. The Ri Kurkas answers that since Shamai says that the skull and spine are Metamei separately even though the Gemara says that we cannot learn out from Shamai because he is stringent, however since we do not find that anyone says differently we follow Shamai's opinion regarding this Halachah. Furthermore all of the other Tum'os mentioned in the Mishnah are Metamei separately and therefore it is likely that the skull and spine are also Metamei separately.
A BONE THE SIZE OF A BARLEY
If a Nazir comes into contact with or carries a bone the size of a barley, he must shave and bring Korbanos Tum'ah and recount his Nezirus. However a bone the size of a barley does not convey Tum'as Ohel. (Rambam Hilchos Nezirus 7:6)