28 Field Museum of Natural History — Zoology, Vol. XL 



In spite of the continually advancing knowledge, due largely to 

 constant discoveries of new forms both living and extinct, there are a 

 great many links inissing in the zoological chain, and it is not surprising 

 that systematists are not entirely in accord in their conclusions and that 

 any system of classification at present evolved, must of necessity be 

 more or less tentative. 



Nearly all modem zoologists now recognize two subclasses of living 

 mammals: I, Prototheria, comprising a single Order, Monotremata, to 

 which belong the Spiny Anteaters {Echidnidce) and Duck-bill {Ornithor- 

 hynchidcB) of the Australian region — strange egg-laying animals which 

 differ anatomically in many ways from other known forms; and II, 

 Eutheria, which includes the rest of the Orders. The Metatheria, con- 

 taining the Order Marsupialia, or pouched mammals, such as the 

 Kangaroos, Opossums, etc., was at one time given rank as a third sub- 

 class but is now generally included with Eutheria.* The subclasses 

 and orders comprising the living mammals of the world are as follows: 



Kingdom ANIMALIA. Animals. 



SuBKiNGDOM VERTEBRATA. Back-boned Animals. 



Class MAMMALIA. Mammals. 



Subclass PROTOTHERIA. 



Order MONOTREMATA. Duck-bill Platypus, Echidnas. 



Subclass EUTHERIA. 



Order MARSUPIALIA. Marsupials or Pouched Mammals. 

 Suborder Polyprotodontia. Opossums, Bandicoots, etc. 

 Suborder Diprotodontia. Kangaroos, Wombats, etc. 



Order EDENTATA. Sloths, Armadillos, etc. 



Suborder Zenarthra. Armadillos, Sloths, Anteaters, etc. 

 Suborder Nomarthra. Aard-varks and Scaly Anteaters. 



Order UNGULATA. Hoofed Mammals. 

 Suborder Proboscidea. Elephants. 

 Suborder Hyracoidea. Hyraxes. 



Suborder Perissodactyla. Horses, Tapirs, Rhinoceros, etc. 

 Suborder Artiodactyla. Pigs, Cattle, Sheep, Antelopes, Deer, Giraffes, 

 Camels, Hippopotami, etc. 



Order SIRENIA. Manatees, Dugongs. 



Order CETE. Whales, etc. 



Suborder Mysticete. Whalebone Whales. 



Suborder Odontocete. Toothed Whales (Sperm Whales, Porpoises, 

 Dolphins, Narwhal, etc.). 



*The absence of an allantoic placenta in all Marsupials has been disproved by 

 its discovery in Parameles. 



