Part of Iggeres Part III Chapter 1
After the death of Ravina, hora'ah was no longer possible, as Shmuel saw written in the Sefer of Adam HaRishon that Ravina and R' Ashi would be the last of the Ba'alei hora'ah. Subsequently, although there was no true hora'ah, there were the Savorai who were close to the level of hora'ah; they also explained obscure pasages, as we find in Gittin 7a... Many of the later Savorai are mentioned in the Talmud, and we have a tradition that the discussion of the Mishna's wording which  opens the Mesechta of Kidushin was inserted by these Savorai.