Hippocampus
The hippocampus has a generally similar appearance across the mammals from monotremes such as the echidna to primates such as humans. The hippocampal size to body-size ratio increases: it is about twice as large for primates as for the echidna. It does not, however, increase at anywhere close to the rate of the neocortex to body-size ratio. Therefore, the hippocampus takes up a larger fraction of the cortex in rodents than in primates.