FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN CORY. 15 



Some zoologists still further abbreviate the formula, giving only one 

 side of the jaw, as the number of teeth on both sides are supposed to be 

 alike, thus: 



I- *> C. - Pm. -> M. - X2 = 5 o. 

 4 i 34 



A reference to a single tooth is often written as follows: (Pm. i) 

 and would mean the first upper premolar, or (Pm. 2), which would refer 

 to the second lower premolar. 



The variation in the number of teeth in different mammals is very 

 great. The Anteaters (Myrmecophagida) have no teeth; the Narwhal, 

 for example, may be said to have but one, although there is another in 

 a rudimentary state imbedded in the upper jaw. Some Rodents have 

 12, others 20; Sloths have 18; Rabbits 28; Man 32; the Armadillo 98; 

 and the Dolphin more than 100. There is also an extraordinary 

 difference in size and shape as, for example, the tusks of the Elephant 

 and the enormously developed tusk or "horn" of the male Narwhal. 



The Vertebral Column, Spinal Column or " Back-bone," consists of a 

 number of bones connected by cartilages. It is generally considered 

 as divided into five regions known as the cervical (neck); dorsal 

 (back), which supports the ribs; lumbar (small of the back); sacral 

 (between the lumbar and caudal), consisting of several vertebrae 

 fused together and supported by the pelvic girdles; and the caudal 

 or bones of the tail (see illustration, p. 10.). The number of verte- 

 bra? varies greatly in different mammals. For example, while in 

 Man the caudal vertebras consist of but three more or less rudi- 

 mentary bones, the tail of the African Scaly Anteater (Manis) has 

 46. 



The Pelvis is the bony framework formed by the pelvic girdles or hip 

 bones and including the sacral vertebrae. 



The Ribs The number of ribs varies in different mammals. They 

 are curved bones which are movably articulated with the dorsal 

 vertebrae, and the upper ones are attached to the sternum with a 

 cartilage which becomes ossified in but few forms. The lower ribs, 

 which are not directly attached to the sternum, are called "floating 

 ribs." 



The Sternum or Breast-bone is a bone, or series of bones (sternebrce) , 

 connected on each side by cartilage with the ribs. In most cases 

 these connections remain cartilaginous during life, ossification 

 occurring in but few cases. The sections of the sternum vary in 

 character and number. The upper part is called the presternum, or 

 manubrium sterni of human anatomy ; the sections below this (except 



