LEPUS 139 



often much alike, but their distinction lies in the fact that the 

 premolars appear in two sets : " milk teeth " arising first and 

 being replaced by permanent teeth as in the case of incisors 

 and canines ; but there is only one generation of molars. 

 Premolars and molars or grinding teeth, are the hindmost teeth 

 to be carried on the maxillae in the upper jaw. 



Bearing in mind the four different sorts of teeth, it is possible 

 to describe the dentition of a mammal very simply and quickly 

 by means of a " dental formula " : that of the rabbit is : 



.2033 



1-, c-, p^,m>. 



1023 



The dental formula of the dog, on the other hand, is : 



.3142 

 v>, c-, p?, m-. 



3 14 3 



Most mammalian teeth grow to a certain size and then 

 cease, as a result of the closing of the entrance into the pulp- 

 cavity by the formation of " roots " or fangs. Some, however, 

 retain the open pulp-cavities which are continuously supplying 

 food material to the odontoblasts, as a result of which the tooth 

 can go on growing throughout life. Such teeth are called 

 " rootless," or " with persistent pulps," and examples are to 

 be found in the incisors of the rabbit. As a rule, teeth which 

 are subjected to perpetual wearing down owing to grinding or 

 gnawing, or which can grow out of the mouth for unlimited 

 distances such as the tusks of the elephant, are of this kind. 

 In the rabbit, the lower incisors are kept in check by the upper 

 ones, and vice versa ; but if one tooth through accident is lost 

 or destroyed, the opposing tooth in the other jaw is no longer 

 resisted in its growth. Under such circumstances it grows 

 continuously and eventually kills the rabbit by preventing it 

 from shutting its mouth. In a sense, it may be compared with 

 the unruly growth of a tumour. 



Skeleton. — The skeleton of mammals has a peculiarity in 

 many of its bones which is not found in any other vertebrates. 

 Several of the cartilage-bones, and especially the vertebrae and 

 the bones of the limbs are composed of three pieces : a central 



