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PHYLUM CHORDATA : CLASS MAMMALIA 



Nevertheless, amid so much uncertainty, we may recall the facts that 

 in Amphibians we find two occipital condyles, a reduced quadrate, a 

 somewhat mammalian carpus, holoblastic ova, and so on. 



The oldest Mammalian fossils are from Triassic strata, but they throw 

 little or no light on pedigree, partly perhaps because they are few and 

 fragmentary, partly also because they seem already speciahsed forms. 

 They are often grouped together as Allotheria or Multituberculata and 

 placed near the Monotremes. 



In the Jurassic period there are more of the dubious Allotheria, 

 e.g. Plagiaulax, some " triconodont " Marsupials, e.g. Triconodon and 

 Amphilestes, and the Trituberculata, e.g. Amphiiherium, some of which 

 suggest primitive Insectivora. There are few Cretaceous fossil remains 

 of Mammals, but some of them suggest that the orders of Eutheria 

 were incipient. 



In the earhest Eocene strata, Mammals related to modern types begin 

 to be abundant, but we cannot do more than notice two points — (a) 

 there were some generalised types, e.g. Creodonts and Condylarthra, 

 with relationships to several extant orders ; (b) that the early forms 

 were mostly small animals with small brains, pentadactyle, with 

 44 teeth, including small canines and bunodont molars. 



Professor Osborn has suggested that there were two main lines of 

 mammalian evolution — {a) the " Mesoplacentaha," e.g. Amblypoda, 

 Coryphodontia, Dinocerata, Tillodontia, and many Condylarthra and 

 Creodonts, in which the brain remained small and unspecialised, which 

 died out in the Miocene (unless the Marsupials, Insectivores, and 

 Lemurs represent their descendants), and {b) the successful lines of 

 " Cenoplacentalia," which made, so to speak, a fresh start, with a 

 premium on brains, and led to most of the modern types. In almost 

 every case, it may be said that an order begins with small repre- 

 sentatives, and that the giant forms almost always indicate the end 

 of a race. 



Systematic Survey of the Orders of Mammalia 



I. Sub-class Prototheria or Ornithodelphia, Orders 



Monotremata, and (?) Allotheria or 

 Multituberculata. 



II, „ Metatheria or Didelphia, Orders Poly- 



protodontia and Diprotodontia. 



III. ,, Eutheria or Monodelphia. 



Orders of Eutheria. 



1. Xenarthra. | .. Ej.^t.tes. 



2. Nomarthra.J 



1. Sirenia. 



