26 



GENERAL ANATOMICAL CHARACTERS 



would be greatly simplified ; but there are so many exceptions that 

 a close scrutiny into the situation, relations, and development of a 

 tooth is required before its nature can be determined, and in some 

 cases the evidence at our disposal is scarcely sufficient for the 

 purpose. In other instances, however, as among the Polyprotodont 

 Marsupials, we have decisive evidence to show that the missing 

 premolar teeth are not those at the extremity of the series. 



The milk -dentition is expressed by a similar formula, d 

 for deciduous or m for milk being commonly prefixed to the 



771.2, 



m. 3 



PIG. 3. Milk and Permanent Dentition of Upper (I.) and Lower (II.) Jaw of the Dog (Canis 

 familiaris), with the symbols by which the different teeth are commonly designated. The third 

 upper molar (m.3) is the only tooth wanting in this animal to complete the typical heterodont 

 mammalian dentition. 



letter expressive of the nature of the tooth. Since the three 

 molars, and almost invariably the first premolar of the permanent 

 series, have no predecessors, the typical milk-dentition would be 

 expressed as follows di f, dc \, dm -f, = -f, total 28. In a few 

 Ungulates, however, such as the Hyrax and Tapir, and in some 

 instances the Rhinoceros and the extinct Palceotherium, the whole of 

 the four premolars are preceded by milk-teeth ; when we have the 

 fullest development of cheek-teeth in the whole of the Eutheria. The 

 teeth which precede the premolars of the permanent series are all 

 called molars in the milk-dentition, although as a general rule, in 



