724 PERISSODACTYLS xxx. 2-3 



Superorder 4. Mesaxonia. Family 2 (cont.) 



America; *Hippidion, Pleistocene, S. America; Equus, 

 horses, asses, zebras, Pliocene-Recent, world-wide 

 * Family 3. Brontotheriidae (= *Titanotheridae). Eocene-Oligo- 

 cene. Holarctic 

 *Lambdotherium, Eocene; *Eotitanops, Eocene; *Brontops, 

 Oligocene 

 *Family 4. Chalicotheriidae. Eocene-Pliocene. Holarctic 



*Eomoropus, Eocene; *Chalicotherium, Oligocene-Pliocene, 

 Eurasia, Africa; *Moropus, Miocene, N. America 

 Suborder 2. Ceratomorpha 

 Superfamily 1. Tapiroidea. Tapirs. Eocene-Recent 



*Homogalax, Eocene; Tapirus, tapir, Miocene-Recent, Asia, 

 S. America 

 Superfamily 2. Rhinocerotoidea. Rhinoceroses. Eocene-Recent 

 ^Family Hyrachyidae. Eocene. Holarctic 



*Hyrachyns. Eocene 

 # Family Hyracodontidae. Eocene-Oligocene. Holarctic 



*Hyracodon, Oligocene 

 *Family Amynodontidae. Eocene-Miocene. Holarctic 



*Amy7iodon, Eocene 

 Family Rhinocerotidae. Oligocene-Recent 



*Aceratherium, Oligocene-Pliocene ; *Balachitherium> Oligo- 

 cene-Miocene, Asia; Rhinoceros, Indian and Javan rhino- 

 ceros, Pliocene-Recent, Asia; Diceros, black African rhino- 

 ceros, Pleistocene-Recent, Africa 



3. Perissodactyl radiation 



The fossil history of animals with the perissodactyl structure is 

 perhaps better known than that of any other mammals; the type 

 reached its peak during a period from which many fossils have been 

 preserved and we have therefore a better opportunity to study the 

 development, flowering, and decay of the group than in the case of 

 forms whose maximum development occurred either earlier or later. 

 Here if anywhere we should be able to learn lessons about the nature 

 of the evolutionary process and to study the forces that produce 

 change in animal form. Because of the very abundance of the fossils 

 it is necessary, however, to be cautious in interpretation and to recog- 

 nize exactly what can be proved from the evidence. 



The known types of horses are divided into 350 species, but only 

 a small proportion of these can be confidently placed close to the 



