Most wickets in a session

I have been unable to find out whether Muttiah Muralitharan's astonishing spell on Monday after lunch is any kind of record. Six wickets in a session by a single bowler in a Test Match? Can't have happened very often since wickets were covered.

If anybody knows a way of comparing this spell with others in history I would be interested to know. I have asked Bill Frindall but no reply yet.