32 MICROSCOPIC ANATOMY OF THE TEETH 



The Evolution of the Human Molar 



Teeth, being often the only perfectly preserved remains of 

 early ancestors of the Mammalia, are of very great impor- 

 tance in the study of the evolution of the various existing 

 forms, and many different views have been held as to the 

 mode in which the highly complicated molars of existing 

 mammals have been evolved from simpler types. 



Although this is a subject which comes more appropriately 

 under the heading of Dental Anatomy, recent researches on 

 certain structures in the enamel organ of Mammalia, which 

 have a strong bearing on the development and evolution of 

 the different forms of teeth, render it necessary to review, 

 however briefly, the principal theories which have been 

 brought forward to account for the origin of these different 

 forms. 



Heterodont When teeth are similar in form and size throughout the 

 and homo- ser i eSj as j n the Dolphin, the dentition is spoken of as 

 homodont ; when, on the other hand, they vary in different 

 parts of the mouth it is described as heterodont. 



Mono- It was stated by Owen that homodont animals usually 



phyodont have but one set of teeth, or are monophyodont. Heterodonts 

 phyodont. possess two sets of teeth, constituting a permanent and a 

 milk dentition, and are spoken of as diphyodont. 



There are exceptions to this rule, however, and it was shown 

 by C. S. Tomes (27 b) that in the Armadillo (Tatusia peba), 

 with a homodont dentition, there are both milk and perma- 

 nent teeth ; it is truly diphyodont, and in many homodont 

 animals traces of a milk dentition have been found, and, as 

 stated by Marett Tims, there are very few animals which 

 can be considered to be truly monophyodont, modern 

 methods of research having shown that in most cases hitherto 

 regarded as monophyodont, functionless representatives of 

 other dentitions are to be found, and the term can only be 

 consistently retained to describe a single functional dentition 

 as in the toothed whales. 



Fish and reptiles are poly phyodont, having a continuous 

 succession of teeth, but in mammals never more than two 

 sets of teeth are developed, although in them also vestiges 

 of additional series have been described by Leche and others . 



