You push in on the back rest at bottom as you pull up on the seat from the crevice
Seeing as I have no idea what this means, I'll describe it in my feeble mind terms...
You need to push your knee into the front vertical face of the seat bottom. (at aproximately the quarter points, maybe a foot from the doors each side) This will 'unhook' the bottom front of the the seat bottom. Then the front of the bottom seat will pivot upward and you can lift it out. Like Bjorn said, the seatbelts may have just fallen down under the seat. There should be three sets of seatbelts for a '68 rear seat, unless a prevoius Owner removed them. If the seat hasn't been out for quite a while, you really have to press hard. I am always afraid I will tear the fabric because of how hard you need to press and lift.
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold