204 MAMMALIAN DENTITION 



TYLOPODA 



Passing next to the Camels and Llamas Ave find that the his- 

 tory of these animals is fairly well known. Their ancestors 

 diverge from those of the Pecora (Deer, Antelopes, etc.) about 

 the middle Eocene and hence the many points of similarity 

 such as the selenodont and hypsodont teeth found in both these 

 groups must obviously be examples of parallel evolution rather 

 than heritages from a common forerunner. 



The history of the Camels and Llamas moreover shows that 

 the development of this group occurred almost wholly in North 

 America, the Camels for some reason at the present unknown 

 ultimately migrating to Asia and the Llamas to South America. 

 The Pecora, on the other hand, are a typically Old-World group. 



Differences between the Camels and Llamas are largely super- 

 ficial, depending upon character of hair, length of ears, general 

 build and the presence in the one group and absence in the 

 other of humps, masses of fat which are naturally unrepre- 

 sented in the skeleton. Apart from size, certain minor dis- 

 tinctions and the absence of the two anterior upper premolars, 

 the skeleton and teeth of the Llama are closely similar to those 

 of the Camel. 



In both animals the skull appears triangular through the 

 abrupt narrowing of the facial region. The orbits are sur- 

 rounded by bony rings and the neck is strikingly long as in 

 the Giraffe. The dental formula in the Camel (Fig. 73) is: 



I-^,C|,P|,M|. total 34. 

 o I , Z o 



The first and second upper incisors are lost very early but 

 the third, the canine and the first premolar are long and sharp 

 teeth capable of giving a very serious bite. The lower incisors 

 are procumbent, large, spatula-shaped and overlapping; the 

 lower canine vertical and standing apart from the incisors. 

 The second premolar is missing and the third and fourth each 

 somewhat resemble the half of a molar ; the third premolar like the 



