The order Perissodactyla is divided into the order Perissodactyla and the order Artiodactyla, which are both categories of mammals. The order Perissodactyla is named because the number of toes is mostly odd. The toes of primitive perissodactyl animals were four in the front and three in the back. Artiodactyla has an even number of hooves, and terrestrial species usually have four or two toes on each of their four legs. They are also described as whale artiodactyls.