DENTITION. 501 



always in a contimious series, but frequently separated by a 

 diastema from the canines. The anterior grinding teeth are 

 called the premolars, the posterior are the molars. The grinding 

 teeth have usually more complex crowns than the anterior teeth, 

 and the premolars have nearly always simpler crowns than the 

 molars. The distinction between them is least marked in herbi- 

 vorous animals, in which the posterior premolars at least resemble 

 the molars. Speaking generally it may be said that there is a 

 serial increase in complexity in proceeding from the front to the 

 hind end of the premolar-molar series. But this does not always 

 apply : in some forms, e.g. many Camivora, the hinder molars 

 are smaller and less complex than the teeth immediately in front 

 of them. The molars are usually distinguished from the pre- 

 molars not only by their more 

 compUcated crowns, but also by 

 the fact that they have no func- 

 tional predecessors in the milk 

 dentition (but are only formed 

 once in the life of the animal). 

 1 his brings us to another charac- 

 teristic feature of the mammalian 

 dentition. Most mammals are 

 diphyodont, i.e. they have two ^^^ ^ ^.^. ^ ^ ^ . ., 



^ ^ ' '' Fig. 265.— Dentition of Cebus while 



functional sets of teeth and changing the teeth (from Glaus, after 



Owen). The small letters point to 

 never more than two sets. Occa- the mllk teeth, the capital letters to 



the permanent teeth. 



sionally there is only one set 



(toothed whales, many edentates, etc.) which last through 

 life : in this case they are said to be monophijodont. When 

 there are two sets, the first set is called the milk or 

 deciduous dentition, because it is usually found in the young 

 animal during the period of active growth. Typically the 

 milk teeth are shed as the jaws attain their full size and are 

 replaced by the permanent dentition (Fig. 265). The first back 

 tooth of the permanent dentition to appear is usually the first 

 molar, which in consequence presents a more worn appearance 

 than the later appearing posterior molars. This frequently 

 gives us a means of determinmg the limit between the premolars 

 and the molars, when the posterior premolars resemble the 

 molars in the form of their crowns, as they sometimes do. 

 A dental formula is a short way of expressing the number and 



