12 PLEISTOCENE MAMMALIA. 



Lydekker's conclusions with regard to the loss of incisors in the hippopotami 

 are : 



(a) That the earliest incisor teeth to disappear are i. 2 and i. 2. 



(/>) That when there is only one pair of incisors remaining-, it is the first. 



(c) That the disappearance of each pair of incisors takes place first in the lower 

 jaw, later in the upper. 



Order of Succession. — The last of the permanent teeth to appear is pm. 4, the 

 last but one is m. 3. The earliest members of the permanent grinding series to 

 appear are m. 1 and m. 1 . 



Permanent Dentition of the Upper Jaw (Plates V and VI). 



The incisor teeth, which are straight, have considerably more than half their 

 length concealed in the alveolus. Enamel is confined to their antero-external 

 surfaces, and with use they assume a chisel-like form from friction with the 

 corresponding teeth of the mandible. The postero-internal face is bevelled 

 away. 



1. 1, which is of even diameter throughout, is only slio-htlv curved, and has the 

 enamel extending over more than a quarter of its surface. A specimen in the 

 Geological Department of the British Museum measures L0"75 cm. in length. 



. I. 2, which is more curved than i. 1, tapers towards its termination, which is 

 somewhat sharply recurved. Here again only a quarter of the surface is coated 

 with enamel. 



C. (Text-fig. 1, a) is a much larger tooth and has nearly four-fifths of the surface 

 enamel-coated, all in fact except the anterior face, which is bevelled away by friction 

 with C. The canine projects further from the alveolus than the incisors. The 

 posterior face of the tooth is marked by a prominent groove, the antero-internal 

 surface by a feeble one. 



The premolars are all conical teeth, and the interval between each and its 

 successor decreases as the series is followed backwards. They vary considerably. 

 and isolated examples of pm. 2 and 3 are difficult to distinguish from pm. 2 and 3. 



Pm. 1 is a small tooth with a simple conical crown and a single root. In a 

 large skull an interval of about 7'5 cm. commonly separates it from C. It is as a 

 rule missing and its alveolus closed in old individuals. As it is cut very early and 

 has no successor it is often considered to belong to the deciduous series and to be 

 dm. 1. 



Pm. 2 is a much larger tooth than pm. 1, with an elongated conical crown and 

 two roots. In some cases there is little indication of a cingulum, in others it is 

 fairly well marked. 



Pm. 3 is a still larger tooth similar in character to pm. 2, but always with a 

 strongly developed cingulum on its posterior and inner borders. 



