276 HISTORY OF THE HUMAN BODY 



The number of each kind of teeth occurring in a given 

 mammal is briefly and graphically expressed by a dental for- 

 mula, which may be written in a variety of ways. Thus the 

 entire dentition may be expressed as in the following formula 

 for the cat, in which the upper and lower rows represent the 

 two jaws, the mid-ventral line is marked by the short per- 

 pendicular and the number of each group of teeth is desig- 

 nated by a digit: 



i-Vi-3 



1-2-1-3 



3-1-2-1 



This formula shows at once that there are in each jaw six 

 incisors flanked on each side by a canine, and that there are 

 four cheek teeth above and three below on each side, only the 

 last one of which is not replaced and is therefore a true molar. 



Except for the sake of symmetry, however, one half alone 

 may be given, as in the following for the Bovidae, the family 

 to which cattle and sheep belong, a formula showing a total 

 loss of canines and of upper incisors : 



003-3 



3-03-3 



From the study of the dentition in all mammals and espe- 

 cially that of marsupials and the more primitive placental 

 mammals, the following hypothetical dentition has been de- 

 duced for the ancestral type, to which all existing dentitions 

 may be referred: 



5-4-T-5 



5-4-1-5 



5-1-4-5 



This formula provides for ten incisors and eighteen cheek 

 teeth in each jaw, with a total of 60 teeth, 30 in each jaw. 

 This formula is nearly attained by some marsupials, where the 

 most primitive condition would be expected, but in placental 

 mammals there is always a reduction of incisors and molars, 

 the former being never more than three upon each side. Thus, 

 in the opossum (a marsupial) the dental formula is: 



