Corner seal sockets (26) are formed at the end of apex seal slots (58) formed in a rotor (10). A twisted split washer spring (70) is positioned in each socket (26) between the base wall (28) of the socket (26) and the inner end of the corner seal (16). The springs (70) bias the outer end surfaces of the corner seals (16) against the inner side surfaces of the housing sidewalls (20, 22). A radial apex seal slot (32) is formed in each corner seal (16). The base (38) of each slot (32) is substantially semi cylindrical in shape. It and the cylindrical peripheral surface (44) of the corner seal (16) form an arch shaped bight that connects side portions of the corner seals (16) together. End portions of the apex seals (18) extend into the apex seal slots (32) in the corner seals (16). The end portions of the apex seal biasing spring (60) contacts spring abutments (52, 54) which project radially inwardly from the outer ends of the apex seal (18).