1 250 CLASS MAMMALIA. 



Among the existing Metatheria the number of true molars is 



generally -, while the premolars are very frequently reduced in 



4 

 number, and there may be five upper incisors. In one existing, 

 and several Mesozoic members of that subclass, the number of true 

 molars exceeds four ; but, with the possible exception of some of 

 these extinct types, there is no known instance of a heterodont 

 Mammal normally having more than four premolars ; and there 

 is never more than a single canine tooth on either side of each 

 jaw. 



Since in Ungulates it is sometimes difficult to distinguish molars 

 from premolars, it may be well to mention how the division between 

 the two series can always be determined. Since the first tooth of the 

 true molar series always comes into use before the last milk-molar is 

 shed, it is obvious that in the adult the first true molar will always 

 be more worn than the last premolar. Thus, in the three teeth of 

 Hipparion represented in fig. 1233, the tooth on the left side of 

 the figure being more worn than the one in the middle is thereby 

 shown to be the first of the true molar series ; the other two being 

 consequently premolars. 



The different types of cheek-teeth will be mentioned under the 

 head of the various orders and families, but a few general observa- 

 tions may be recorded here. Professor Osborn considers that the 

 primitive Mammals had simple conical teeth with undivided fangs, 

 and that the crowns of the teeth of the upper and lower jaws mu- 

 tually interlocked. Teeth nearly approaching to this type occur in 

 the Triassic Dromatherium (fig. 1140), while those of the Dolphins 

 are looked upon as a reversion to this type. Another simple type, 

 according to the views of the same author, is that found in Tricono- 

 don and Priacodon (fig. 1147), where the upper and lower teeth 

 alike consist of three cusps in a line ; the upper teeth biting on the 

 outer side of the lower. A third common, and apparently very 

 generalised, type of tooth is that known as the tritubercular. This 

 consists in the upper teeth of one inner and two outer cusps, ar- 

 ranged in a triangle ; while in the lower jaw the reverse arrangement 

 obtains, so that there is one cusp on the outer and two cusps on the 

 inner side of the crown. An example of this type of structure in 

 its simplest form occurs in the Mesozoic genus Spalacotherium. 

 Modifications of this type occur in the lower teeth of many Marsu- 

 pials {e.g., fig. 1 145), and also in the lower carnassial teeth of the 

 Carnivora, of which mention is made in the sequel. The trituber- 

 cular type of tooth is regarded by the American Palaeontologists as 

 one which has given rise to a large number of the more complex 

 modifications ; and it is extremely common among the generalised 

 Mammals of the Lower Eocene. 



