1. IN A CORE BARREL, THE COMBINATION COMPRISING: (A) AN OUTER DRILL TUBE HAVING A DRILL BIT AT ITS LOWER END AND ADAPTED TO BE ROTATABLY DRIVEN AT ITS UPPER END, (B) SAID DRILL TUBE HAVING AN INTERNAL GROOVE DISPOSED ABOVE ITS LOWER END AND FORMING LOWER AND UPPER SHOULDERS, (C) SAID DRILL TUBE ALSO HAVING A PORTION OF REDUCED DIAMETER FORMING A RIGID VALVE SURFACE, (D) AND AN INNER TUBE ASSEMBLY HAVING SUBSTANTIALLY PARALLEL WALLS THROUGHOUT ITS LENGTH AND ADAPTED TO BE DISPOSED WITHIN SAID DRILL TUBE NEAR THE BOTTOM THEREOF, (E) SAID INNER TUBE ASSEMBLY BEING OF LESSER OUTSIDE DIAMETER THROUGHOUT THAN THE INSIDE DIAMETER OF SAID DRILL TUBE TO PROVIDE A SPACE FOR FLUID FLOW THEREBETWEEN, WITH SAID ASSEMBLY COMPRISING: (1) A LATCH BODY HAVING A TUBULAR WALL, (2) RADIALLY MOVABLE LATCH MEANS MOUNTED TO SAID LATCH BODY AND EXTENDING THROUGH THE WALL THEREOF, (3) MEANS DISPOSED WITHIN SAID LATCH BODY TO BIAS SAID LATCH MEANS RADIALLY OUTWARDLY AGAINST THE INNER WALL OF SAID DRILL TUBE SO THAT SAID LATCH MEANS WILL ENTER SAID GROOVE AND HANG SAID ASSEMBLY FROM SAID LOWER SHOULDER, (4) MEANS COUNTERACTING SAID BIASING MEANS FOR RETRACTING SAID LATCH MEANS FROM SAID GROOVE TO PERMIT WITHDRAWAL OF SAID ASSEMBLY UPWARDLY IN SAID DRILL TUBE, (5) A VALVE MEMBER HAVING A RIGID VALVE SURFACE NORMALLY SPACED FROM SAID FIRST-NAMED VALVE SURFACE WHEN SAID LATCH MEANS IS DISPOSED IN SAID GROOVE,