502 MAMMALIA. 



kinds of teeth present. In the normal complete mammahaii 

 dentition, which- is found in but few Uving mammals (e.g. Sus, 

 Gymnura, TaJpa. Myogale) there are on each side and in each jaw 

 three incisors, one canine, four premolars, and three molars. 

 This is exj^ressed by the formula : ?' f c \ p ^ m |- = 44. 

 In such a case the deciduous dentition would consist of three 

 incisors, one canine, and four molars on each side in each 

 jaw and the formula would be written thus : di ^ dc \ dm | 

 = 32. Frequently however the first premolar in the complete 

 mammalian dentition has no predecessor in the milk series (a few 

 ungulates and Hyrax may be mentioned as cases in which there 

 are four milk molars), so that the milk dentition would be. di |- 

 dc Y dm f =28. Sometimes the dental formula is written more 

 fully, so that each tooth receives its special number, thus : — 

 * r44' c \ r> ]' "' t "* m r^' This method of writing the formula 

 enables us to express our views shortly as to the homologies 

 between teeth in different animals, in which the full number of 

 teeth are not formed. Thus the first premolar of the permanent 

 dentition, the tooth which frequently has no predecoss^or in the 

 milk dentition (it is disputed however whether this tooth belongs 

 to the milk or to the permanent series) often falls out very early. 

 We could express its absence thus : 'p "' 'l ^' ;|; 



Such determinations must not however be pressed too far, as 

 it is usually quite impossible to determine homologies between 

 teeth of the same category in different dentitions, as has been 

 shown by Bateson,* and sometimes it is even difficult to deter- 

 mine whether a tooth as distinctive as the canine is present or 

 absent. The canine tooth in the upper jaw is the first tooth in 

 the maxillary bone, and the canine in the lower jaw is the tooth 

 which bites in front of this. But as the first premolar may 

 simulate a canine in appearance and the real canine may simulate 

 an incisor, it is sometimes difficult to be certain as to whether 

 we are dealing with a canine or not. The canine may be looked 

 upon as the first tooth of the premolar series ; it is usually but 

 not always enlarged and caniniform in appearance. 



If it is difficult in doixbtful cases to settle whether a tooth so distinctive 

 as a canine is present, miich more difficult must it be to decide which tooth 

 of a given category is absent, in cases in which the full number is not pre- 

 sent. It is usual to assume that the incisors are diminished from the 

 posterior end, i.e. if there are only two, it is the 3rd incisor which has gone. 



* Materials for the Study of Variaiwn, London, 1804. 



