Tosefta for Nazir 52:5

A quarter-kav of bones from the majority of the skeleton in size, and bones of (i.e., comprising) the majority of the skeleton in size, even though they do not contain a quarter-kav, are impure. A quarter-kav of bones from the majority of the skeleton in quantity, and bones of the majority of the skeleton in quantity, even though they do not contain a quarter-kav, are impure. Rabbi Yehuda stated a different version [of the foregoing halacha]: Beit Shammai say, a quarter-kav of bones from the skeleton from the structural majority or from numerical majority, [or] the structural majority or numerical majority of a corpse (i.e., considering the limbs as opposed to the bones), even though they do not contain a quarter-kav, are impure. And what is its "structural [majority]"? The calves and the thighs, and the ribs, and the spinal column. And what is its "numerical [majority]"? The digits of its hands and its feet, as long as they comprise [one hundred] twenty-five.^*There are many variants to the above; this translation generally follows Rav Shimon of Sens. Note that Mishnah Oholot 1:8 states that there are six bones per finger/toe (thirty per hand/foot), yielding 120; Beit Shammai may have had a different opinion or the received text may be corrupt. Said Rabbi Yehoshua, I can reconcile the words of Beit Shammai and Beit Hillel [as follows]: One from the calves and from the thighs. It comes out to be the structural majority in size; and half of the numerical majority, and half of the structural majority do not join together [for purposes of impurity]. The equivalent of half of an olive's-bulk of flesh and half of an olive's-bulk of decomposed corpse matter do join with one another, and the remainder of all the impurities from a corpse do not join with one another, because their measurements are not equivalent.