The Time Before History, Colin Tudge, Simon and Schuster, 1997, ISBN 978-0-684-83052-0, ... Three main groups of ungulates prevail today: the perissodactyls, which carry their weight on their middle toes like horses, rhinos, and tapirs; the artiodactyls - including the pigs, hippos, camels, and ruminants - which bear their weight equally on their third and fourth toes (and hence are "cloven-hoofed"); and the proboscideans, represented by the elephants ...
