680 MAMMALIA. 



e.g. Plagiaulax, Tritylodon, Polymastodon, should be ranked beside the 

 Monotremes. 



Other Mesozoic forms, such as Dromatherium, Triconodon, Amphi- 

 testes, are often referred to the Marsupial series beside opossum, dasyure, 

 and bandicoot. 



The first certain remains of Eutherian Mammals are found in Eocene 

 strata, and give evidence of the existence of generalised types connecting 

 rather than referable to the modern orders. Many are characterised by 

 the presence of three tubercles on the back teeth, and of five digits on 

 the limbs, and by having brains relatively smaller than those of their 

 modern successors. 



Among extinct Tertiary types we may especially notice the 

 ground sloths (e.g. Megatherium) and Glyptodonts allied to the 

 modern Edentata, the Zeuglodonts sometimes included among 

 Cetaceans, numerous ancestral Ungulates, and the Cieodonts allied 

 to modern Carnivores. 



As already noticed, the recent discovery of an allantoic placenta in 

 Perameles, and the researches on the teeth of the same genus, suggest 

 that the Eutheria and Metatheria arose from a common stock ; but the 

 relation of these sub-classes to the Monotremes remains uncertain. 



Systematic Survey of the Orders of Mammalia. 



/ I. Sub-class Prototheria or Ornithodelphia, Orders 

 * Monotremata, and Allotheria or Multi- 



tuberculata. 



II. „ Metatheria or Didelphia, Orders Poly- 



protodontia and Diprotodontia. 



III. „ Eutheria or Monodelphia. 



Orders of Eutheria. 



i. Edentata. 



2. Sirenia. 



3. Ungulata. 



Pe r rL°sodactyia. } Ungulata Vera. 



Hyracoidea. 



Proboscidea. 



Extinct sub-orders. 



4. Cetacea. 



Mystacoceti — baleen cetaceans. 

 Archaeoceti — (extinct types). 

 Odontoceti — toothed cetaceans. 



