Image source: Gray's Anatomy (1918 ed.)
The hippocampus is a part of the vertebrate brain consisting of two ridges, one over each of the two lateral ventricles. It is highly developed in advanced mammals (primates and whales) and is involved in the early stages of memory formation. It is one of the first regions of the brain to be damaged in Alzheimer's disease. The hippocampus and the amygdala are part of the limbic system.