xxviii. 3 CLASSIFICATION 699 



by a flattened facet, allowing rotatory action of the lower jaw. The 

 pterygoid and masseter muscles are well developed, the temporal less 

 so and the skull is flat and without a sagittal crest, in contrast with 

 carnivores. To provide lateral attachment for these muscles there 

 is a tendency for a redevelopment of the post-orbital bar. 



In the digestive system of ungulates there is usually some chamber 

 in which bacterial action upon cellulose can take place, but this has 

 evidently evolved independently in the different groups, being in the 

 stomach of artiodactyls but in the caecum of perissodactyls. 



This set of 'ungulate' characteristics has developed independently 

 many times in descendants of the insectivoran eutherian ancestor, 

 and shows strikingly how the adoption of a particular method of life 

 leads to selection of variations of structure tending in similar direc- 

 tions. There is no special difficulty in understanding how this has 

 happened if we imagine that each part varies genetically in its dimen- 

 sions. A herbivorous diet will be easier for animals with ridged teeth 

 and long legs, whereas those with sharper teeth can become carni- 

 vores. Types are selected that combine a nervous organization leading 

 to certain habits with other features that make these habits successful. 

 In the evolution of any population there is evidently an elaborate inter- 

 play between variation in different directions in various organ systems 

 and changes in the environmental circumstances. 



3. Classification 



Superorder 2. Protoungulata 

 *Order 1. Condylarthra. Palaeocene-Eocene 



*Family 1. Hyopsodontidae. Palaeocene-Eocene. N. America 



*Mioclaenus, Palaeocene; *Hyopsodiis, Eocene 

 *Family 2. Phenacodontidae. Palaeocene-Eocene. Holarctic 



*Tetraclaenodon, Palaeocene; *Phenacodns, Palaeocene-Eocene 

 * Family 3. Didolodontidae. Palaeocene-Miocene. S. America 



*Didolodns 

 *Family 4. Periptychidae. Palaeocene. N. America 



*Periptychus 

 *Family 5. Meniscotheriidae. Palaeocene-Eocene 

 *Meniscotherium, Holarctic 

 *Order 2. Notoungulata. Palaeocene-Pleistocene 



*Palaeostylops, Palaeocene, Asia; *Notostylops, Eocene, S. 

 America; *Toxodon, Pleistocene, S. America; *Homalo- 

 dotherium, Miocene, S. America; *Hegetotherinm, Oligo- 

 cene-Miocene, S. America 



