5. IN CORING APPARATUS INCLUDING AN ANNULAR DRILL BIT CONTAINING PORTS THROUGH WHICH DRILLING FLUID IS DISCHARGED AND A CORE BARREL PROVIDED WITH AN OUTER HOUSING CONNECTED TO SAID BIT, A CONCENTRIC INNER MEMBER EXTENDING DOWNWARDLY IN SAID HOUSING NEAR SAID BIT, AND A PASSAGEWAY BETWEEN SAID INNER MEMBER AND SAID HOUSING THROUGH WHICH FLUID IS CIRCULATED DOWNWARDLY TO SAID PORTS IN SAID BIT, THE IMPROVEMENT WHICH COMPRISES: (A) A SLEEVE EXTENDING BETWEEN SAID BIT AND SAID INNER MEMBER IN SAID CORE BARREL, SAID SLEEVE BEING AXIALLY SLIDABLE WITH RESPECT TO SAID BIT AND INNER MEMBER IN RESPONSE TO AN INCREASE IN PRESSURE WITHIN SAID PASSAGEWAY IN SAID BIT AND CORE BARREL; (B) MEANS FOR MAINTAINING A SEAL BETWEEN SAID SLEEVE AND BIT; (C) MEANS FOR MAINTAINING A SEAL BETWEEN SAID SLEEVE AND INNER MEMBER; (D) MEANS FOR NORMALLY BIASING SAID SLEEVE IN A FIRST AXIAL POSITION WITH RESPECT TO SAID BIT AND INNER MEMBER AND PERMITTING MOVEMENT OF SAID SLEEVE INTO A SECOND AXIAL POSITION WITH RESPECT TO SAID BIT AND INNER MEMBER IN RESPONSE TO AN INCREASE IN PRESSURE WITHIN SAID PASSAGEWAY; AND, (E) RETAINING MEANS CONNECTED TO SAID SLEEVE AND MOVABLE THEREWITH, SAID RETAINING MEANS CONTACTING THE SURFACE OF A CORE EXTENDING THROUGH SAID BIT INTO SAID CORE BARREL WHEN SAID SLEEVE IS IN SAID FIRST AXIAL POSITION AND EXTENDING AWAY FROM THE SURFACE OF SAID CORE WHEN SAID SLEEVE IS IN SAID SECOND AXIAL POSITION.