28  Frectp Museum or Naturat History — Zodzoey, Vor. XI. 
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 missing in the zoélogical 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 modern zodlogists now recognize two subclasses of living 
mammals: I, Prototheria, comprising a single Order, Monotremata, to 
which belong the Spiny Anteaters (Echidnide) and Duck-bill (Ornithor- 
hynchid@) 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: 
Kincpom ANIMALIA. Anmats. 
SUBKINGDOM VERTEBRATA. Back-BoNED ANIMALS. 
CLtass MAMMALIA. Mammats. 
SuscLtass PROTOTHERIA. 
Order MONOTREMATA. Duck-bill Platypus, Echidnas. 
Suscrass 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 i 
its discovery in Parameles. @ been disproved by 
