We have investigated Focal Conic Domains (FCD) in several materials that have a transition from a SmA to a nematic phase (N): the focal lines are seldom an ideal ellipse and an ideal hyperbola, and suffer large transformations (in shape and in size) when approaching the transition to the nematic phase, or appear imperfect on cooling from the nematic phase. Some examples of these transformations are shown, including the temperature dependence of the ellipse diameter. Contrariwise, FCDs remain unchanged on heating up to or cooling down from the transition to the isotropic phase in materials that have a direct transition to the isotropic phase. We interpret the imperfections of the FCDs observed in nematogenic materials as due to the interactions of FCDs with dislocations. Such interactions are mediated through the presence of kinks on the focal lines of the FCDs, to which the dislocations are attached.