bottom opening 208 the ball head 122 of the screw 120 is insertable into the socket 206 and is initially polyaxially rotatable within the socket through a wide range of angles which are limited only by the contact of the neck 124 of the screw against the lip 210 of the opening 208 the diameter of the neck 124 necessarily being less than that of the opening 208 the upper portion 204 of the cross bar mounting element 200 comprises a pair of upwardly extending members 212a 212b which define therebetween a u shaped trough 214 the front face of the upper portion 204 the front faces 216a 216b of the upwardly extending members 212a 212b includes a series of radially extending ridges 218 which provide a splined conformation thereon referring now also to fig7 and 8 lateral side and bottom views of the cross bar mounting element 200 are provided the lower socket portion 202 which includes the interior socket volume 206 necessarily includes a means by which the socket opening 208 may be expanded this means comprises a vertical split or slot 220 which extends upwardly from the bottom of the element 200 into the upper portion more specifically the split 220 provides opposing slots.