Rosh Hashanah, Chapter 3, Mishnah 2. From here on the Mishnah no longer deals with the laws of sanctifying the new month, but turns to the mitzvos connected with Rosh Hashanah, foremost among them the mitzvah of shofar.
The text of the Mishnah:
"Kol hashofaros ksheirim chutz mishel parah, mipnei shehu keren" - all types of shofaros are valid for the mitzvah, except for the horn of a cow, which is not called a shofar but only a keren (horn).
Among the valid shofaros:
The horn of a ram - the curved horn, which is the most common.
The shofar of a yael - the yael is a species similar to a wild goat, and its horn is straighter.
The opinion of Rabbi Yosi:
Rabbi Yosi disagrees and holds: "Vahalo kol hashofaros nikre'u keren, shene'emar 'bimshoch b'keren hayovel'" - the verse calls even a horn made from the horn of a ram "keren hayovel" ("yovel" refers to a ram). In his view, the very name "keren" does not disqualify, and therefore the horn of a cow should be valid as well.
The response of the Sages:
The Gemara explains that the Sages answered Rabbi Yosi: it is true that all shofaros are also called "keren," but they are called "shofar" as well; the horn of a cow, however, is called only "keren" and is never called "shofar" at all. This is what the Mishnah means when it says "mipnei shehu keren" - because it is called a keren exclusively.
An additional reason - "ein kateigor na'aseh saneigor":
The Gemara adds a further reason for the disqualification, based on the principle that the prosecutor does not become the defense attorney: since the cow was involved in the sin of the Golden Calf, we do not want any involvement of a cow on Rosh Hashanah, at the very time that we are asking to be remembered for good and to have our sins forgiven.
In summary: in this Mishnah we learned that all shofaros are valid - the horn of a ram and the horn of a yael - except for that of a cow, since it is called only "keren" and not "shofar"; we saw the opinion of Rabbi Yosi from the verse "b'keren hayovel," the response of the Sages distinguishing between an exclusive name and a shared name, and the additional reason of "ein kateigor na'aseh saneigor" because of the sin of the Golden Calf.