578 PLACENTAL MAMMALS xxi. 9 



Classification (cont.) 



Cohort 3. Mutica 



Order Cetacea 

 Cohort 4. Ferungulata 

 Superorder 1. Ferae 



Order Carnivora 

 Superorder 2. Protungulata 



*Order 1. Condylarthra 



# Order 2. Notoungulata 



* Order 3. Litopterna 



*Order 4. Astrapotheria 



Order 5. Tubulidentata 

 Superorder 3. Paenungulata 



Order 1. Hyracoidea 



Order 2. Proboscidea 



# Order 3. Pantodonta 



# Order 4. Dinocerata 



*Order 5. Pyrotheria 



*Order 6. Embrithopoda 



Order 7. Sirenia 

 Superorder 4. Mesaxonia 



Order Perissodactyla 

 Superorder 5. Paraxonia 



Order Artiodactyla 



For purposes of phylogenetic study as well as classificatory con- 

 venience it is desirable to attempt to discover how the original 

 eutherian population became divided at its Cretaceous origin, and 

 whether there were main trunks of the placental tree. By Eocene 

 times most of the existing orders were already well established (Fig. 

 356) and it is often stated that the branching of the population occurred 

 relatively rapidly, though it is doubtful if we may use this term for a 

 process occupying perhaps 30 million years during the late Cretaceous 

 and Palaeocene! This early expansion into varied branches, occurring 

 at a time when few fossils were being formed, makes it difficult to 

 discover the outlines of the main divisions. Nevertheless, careful 

 piecing together of evidence suggests grouping of the twenty-six 

 eutherian orders into four main cohorts, and the division corresponds 

 in the main to the classification originally proposed by Linnaeus in 

 1766 on a basis of the foot structure. The Unguiculata include orders 

 in which the original characteristics of the mammalian type have been 



