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 
Fia. 3.—Milk and Permanent Dentition of Upper (I.) and Lower (II.) Jaw of the Dog (Canis 
Jamiliaris), 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 8, de 1,dm 8,=4, total 28. In a few 
Ungulates, however, such as the Hyrax and Tapir, and in some 
instances the Rhinoceros and the extinct Paleotheriwm, 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 
