Ammon's horn A part of the brain consisting of a three layered cortex located in the forebrain bordering the medial surface of the lateral ventricle. The term hippocampus is often used synonymously with hippocampal formation which consists of the hippocampus proper or Cornu Ammonis, the dentate gyrus and the subiculum. Ammon's horn regional part of brain structure with developmental contribution from neural crest