130 Prof. H. G. Seeley. On the Nature and Limits [Apr. 2(5, 



reptilian elements in the skeleton, would still preserve teeth which 

 might be compared with teeth of reptiles ; and as a matter of observa- 

 tion it is found that there are several features in which teeth of 

 reptiles and mammals resemble each other morphologically. 



The idea conveyed by the expression " mammalian tooth " is 

 necessarily that specialisation of tooth structure which is limited to 

 the mammalian class. It may be unknown in the dental conditions 

 of entire families and orders of mammals. And there is an absence 

 of pronounced character in the incisor or canine teeth of any mammal 

 order which would distinguish them as mammalian. 



Similarly the idea implied in the term "reptilian tooth" is the 

 specialisation of teeth in the reptilian class, which is as far from being 

 universal in the class, as mammalian teeth are universal among 

 mammals. Indeed, the lower mammals emphatically approach 

 towards reptiles in all essential characters of tooth form. 



Because the diversities in the teeth of the two classes have been 

 emphasised for purposes of classification, the significance of the 

 resemblances has been less considered. 



There are six typical characters of teeth which are regarded as 

 mammalian. They are : — 



(1.) The presence of more than one root to a tooth; 



(2.) The implantation of teeth by distinct sockets ; 



(3.) The existence of different kinds of teeth in the same jaw ; 



(4.) The development of distinct cusps to the teeth; 



(5.) The wear of the crown with use ; 



(6.) Replacement by a successional series ; 



No one of these characters can be relied on as constant in the class; 

 and its loss is in every case an approach towards a reptilian type. 



First, the root is not the original or essential part of the tooth. 

 While the sucessional teeth are within the jaw they commonly have 

 the roots undeveloped, and thus up to a certain stage of growth are 

 without this evidence of class character. There is never more than 

 one root to an incisor or canine tooth in any mammal ; and never more 

 than one root to any tooth (so far as I can ascertain) in an existing 

 Edentate or Cetacean. Hence if all mammals are supposed to have 

 had a common origin, it is legitimate to conclude that all the teeth 

 originally possessed but one root ; and that there is a certain relation 

 subsequently established between the complexity of the crown and 

 the number of the roots. 



The situation of a root would imply that its development is due to 

 the same law of growth under intermittent pressure or strain as deter- 

 mines the form or elongation of any other bone.* If more than one 

 root is present they are commonly beneath the several parts of a tooth 

 which have to resist intermittent strain or pressure. If the pressure 

 * " The Mechanism of Growth," ' Ann. Mag. IN'at. Hist.,' April, 1S72. 



