And the noun קשקשת (fish scales)¹²⁷ according to the anonymous first opinion (in Sifra) is a singular noun, likeצנצנת (jar). And according to Rabbi Yehuda, it is a collective noun, while the singular form of the noun is קשקש – he reasoned that if it were a singular noun, it would have been written in plural as קשקשות, not קשקשים.

footnotes: ¹²⁷ Leviticus 11:9.

And there are nouns that will always come in plural, for example ערבי נחל (willows of the brook). And see more in chapter 33; for nouns צאן כבשים עזים (sheep, lamb, goats), etc., there are many rules pertaining to this matter.