1. In a polyaxial bone anchor, the improvement comprising: a) a receiver defining a chamber communicating with a channel, the channel sized and shaped for receiving a portion of a longitudinal connecting member, the chamber communicating with a lower opening and further comprising an aperture defined by a first inner surface; b) a shank having a body and an upper curved portion having a hemisphere, the shank body extending through the receiver lower opening; and c) a resilient open retainer located in the chamber, the retainer having a base and an upper structure, the base expandable in the chamber about the shank upper portion and receiving the shank upper portion therethrough to capture the upper portion in the chamber, the retainer upper structure having a top surface that is located on the top of the retainer and being disposed in the aperture in the chamber of the receiver and the retainer base being fixed against a cylindrical portion of the chamber of the receiver when the shank is in a locked position with respect to the receiver, the shank upper portion being pressed against the retainer base and being in spaced relation with the receiver when in the locked position.
6. The improvement of claim 1 wherein the retainer base has a first planar surface seated on a second planar surface partially defining the receiver chamber.