38 ILLINOIS NATURAL HISTORY SURVEY MANUAL 4 



adult a backbone or a sejimented spinal column. The most prim- 

 itive vertebrates, which include some eel-like forms and the 

 fishes, live in water. Many of the more hij:;hly specialized forms 

 of vertebrates, at least in the adult stage, have four feet or 

 limbs instead of fins. Some of the familiar groups of Vertebrata, 

 called classes, are as follows: 



Class CHONDRICHTHYES— sharklike fishes 



Class OSTEICHTHYES— bony fishes 



Class AMPHIBIA — salamanders and frogs 



Class REPTILIA — turtles, lizards, crocodiles, and snakes 



Class AVES— birds 



Class MAMMALIA — mammals 



The Mammalia comprise three groups very different in struc- 

 ture and habits: One group, the egg-laying mammals or Mono- 

 tremata, is confined to the Australasian Region. The second 

 group, the pouched mammals or Marsupialia, is primarily Aus- 

 tralian and South American in distribution but is represented 

 in Illinois by a single species, the opossum. The third group, 

 the placental mammals or Eutheria, is the dominant mammal 

 group in most parts of the world and to it belong all the species 

 of the Illinois fauna except the opossum. 



Members of the class Mammalia are not only abundant but 

 are also of many different kinds, such as bats, mice, deer, wolves, 

 elephants, and whales. The most closely related kinds are 

 grouped together into families, and related families are grouped 

 together into orders. The 59 species of wild mammals known 

 to occur in Illinois represent 7 orders and 16 families. Some 

 well-known orders not found in Illinois include the whales and 

 porpoises (order Cetacea), which occur only in the ocean; the 

 sloths and armadillos (order Edentata), which occur in the 

 American tropics and as far north as Kansas; and the manatees 

 and sea cows (order Sirenia), which occur in seas and estuaries 

 in Florida and many other parts of the world. 



Checklist of Present Native and Naturalized Mammals 



of Illinois 



Order MARSUPIALIA 



Family Didelphidae — opossums 



Didelphis marsupialis Opossum 



