INTRODUCTION. 



A. The Proboscidea. 



Since the cheek-teeth of the Dinot7ieriidce do not differ in their mode 

 of succession from those of the Perissodactylate and Artiodactylate 

 suborders of the Ungulata, there can be no hesitation in employing 

 the same system of enumeration ; and the two premolars are accord- 

 ingly respectively termed pm. 3 and pm. 4, and the milk-teeth which 

 they replace mm. 3 and mm. 4. This being so, it will be obvious 

 that the corresponding teeth of the Elephantidce must also be num- 

 bered in the same manner. The first cheek-tooth which normally 

 occurs in the Elephantidce appears to be serially homologous with 

 the two following teeth, and is accordingly reckoned as a milk-molar 

 (mm. 2), although there is apparently only one known instance of 

 its replacement by a premolar 3 ; and hence the similarly-placed 

 tooth which occurs in the lower jaw of DinotTierium 2 is also classed 

 with the milk-series. In the absence of any evidence to the con- 

 trary, the so-called preantepenultimate milk-molar, which it is be- 

 lieved is occasionally developed in the Elephantidce, is referred to 

 the milk-molar rather than to the premolar series 3 . In the Dino- 

 iheriidce, where the whole of the permanent cheek-series of teeth 

 is in use at the same time, and where no one tooth attains excessive 

 development, the third and fourth premolars attain the same rela- 



1 Infra, p. 13. 2 Infra, p. 3. 



3 The author is inclined to believe that the first cheek-tooth in the Perisso- 

 dactyla — which in Tapirus is always replaced by a vertical successor, in Rhino- 

 ceros is occasionally so replaced, but in Equus never has any successor, and is 

 frequently absent — belongs to the milk-molar rather than to the premolar 

 series. 



