The hippocampal formation consists of three contiguous long (about 8 cm long) strips of cortex at the ventromedial edge of the temporal lobe cortex. The 3 strips are 1) the hippocampus (proper) 2) the dentate (gyrus), and 3) the subiculum. They are curled up, roughly like an “S”, and best conceptualized by straightening or unfolding the “S”: the hippocampus is the middle strip, the dentate gyrus borders the inner/medial edge of the hippocampus, and the subiculum borders the outer edge of the hippocampus. The subiculum separates the hippocampus proper from the entorhinal neocortex of the parahippocampal gyrus (there is nothing at the other edge of the dentate). The dentate and hippocampus are primitive, 3-layered archicortex, and the subiculum is transitional cortex.
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