SUCCESSION OF MAMMALIAN TEETH. 441 



(11), who believes that if Thomas' pm2 exists it proves the 

 presence of five, not four, premolars in the marsupials, as 

 he considers with Owen (4) that the first so-called molar of 

 these forms is really the last premolar. Continental obser- 

 vers (26, 32) persist in calling this changing tooth pn-13, 

 because they have been unable to find any trace of the miss- 

 ing pm2 of Thomas in the developmental stage. While 

 studying the development of the premolars in the Macropi- 

 didce (12) I have deduced reason for the belief that the 

 successional premolar is in reality not a replacing tooth but 

 a retarded milk tooth, intermediate in position between 

 pm.3 and pm4, and I further discovered the presence of 

 one premolar germ in front of the functional cheek teeth. 

 This would give us four premolars, without counting the 

 first molar as do Cope and Owen. 



The condition of this replacing tooth in Macropus might 

 well appear to support the theory of Baume (13) that the two 

 dentitions of the mammalia had a common origin in a single 

 set lineally disposed, and that each alternate tooth, owing to 

 the shortening of the jaw, became retarded in its develop- 

 ment and displaced so as to be situated below and behind 

 the tooth immediately mi front, thus giving rise to the. two 

 apparently distinct generations of teeth, the underlying ones 

 eventually pushing out the more superficial set. 



This theory might explain to a certain extent the rela- 

 tion of the teeth seen in some reptiles (i.e., Varanidce) and 

 also the case of the premolars of Macropus just mentioned, 

 but we have no palseontological evidence which in any way 

 bears out this view in the mammalia and I should rather re- 

 gard the case above recorded as a feature developed in the 

 marsupials alone, i.e., one in which a milk tooth has simu- 

 lated the relations of a replacing tooth, and not as one which 

 gives us any real clue to the relations of the milk and 

 replacing dentitions. 



All attempts to study the homologies of the individual 

 teeth in different orders of mammalia are very important 

 from a phylogenetic point of view, and we owe much to 

 Thomas (10) for his careful study and suggestions in this 

 direction. Unfortunately even now we know very little of 



