ArchiEopteryx, the earliest of the true birds, belonging 

 to the Upper Oolite. 



All of the early Ichtliyopsidans and Sauropsidans 

 were more or less mammalian in character. The dif- 

 ference between the hand-footed Cheirotherium, which 

 left its tracks in the mud of the Lower Trias ; the 

 Labyrinthodonts and other Amphibians ; the Proto- 

 rosaurs of the Permian, and other Triassic Saurop- 

 sida, and the Mammals, was not so great as might be 

 anticipated by comparing modern representatives of 

 these ancient forms. At tlie time when the mammals 

 appeared with characters borrowed from both the 

 great groups which preceded them, the four lateral 

 branches of fishes, amphibians, reptiles, and birds had 

 not attained their distinctive characters. These were 

 the result of later development. 



The class of Mammajja branches into three main 

 divisions, known as Orxithodelphia, Didelphia, and 



MONODELPHIA. 



The Ornithodelphia, or Monotremata, are the lowest 

 and most bird-like of Mammals. They combine both 

 Sauropsidan and Ichthyojj-^idan characters with those 

 which distinguish the gi'oup to which they belong. 

 They consist of two families only, each represented by 

 a single genus, and belong to Australia. They are 

 Echi<h)(i^ or the Porcupine Ant-eater, of the family 

 Tachif gloss idee ; and Orniflio/ hipichus. Platypus, or the 

 Duck Mole, of the family ()rnitIiorlu/nc}iid(2. No fossil 

 OrmOwdelph'ia are known. 



The DiDELPHiANS, or Marsupials are also a low grade 

 of Mammals, with imperfect young. If divided ac- 

 cordinu" to the nature of their food, they constitute 



