GENERAL CHARACTERISTICS. 



J 55 



A LEFT UPPER MOLAR TOOTH OF THE NILGAI, 

 SEEN FROM THE INNER AND OUTER 

 SIDES. 



stages of wear. This will be apparent from a comparison of the accompanying 

 figures, the first of which shows a tall-crowned tooth viewed from the inner and 

 outer sides, while the second shows a short-crowned tooth seen directly from above. 

 In the former the valleys between the four 

 crescent - shaped columns form deep pits, 

 penetrating the whole extent of the crown 

 of the tooth, while in the latter they are mere 

 shallow channels. It will be found that while 

 all the earlier Ungulates have short-crowned 

 cheek - teeth, the greater number of living- 

 species have high-crowned ones ; and it will 

 also be observed later on that the develop- 

 ment of high-crowned teeth has taken place 

 independently in each of the four great groups 

 into which existing Ungulates are divided. 

 It should also be mentioned that whereas in 

 Carnivores the upper molar teeth are gener- 

 ally of the primitive triangular type, in all 

 existing Ungulates they have assumed the 



quadrangular form. The food of the Ungulates consisting in most cases entirely 

 of vegetable substances requiring much mastication, is the inducing cause for the 

 complex structure of the cheek-teeth in the more specialised kinds ; and to the same 

 cause may be attributed the circumstance that Ungulates always retain the full 

 number of molar teeth, and, except in the camels, at least three out of the typical 

 four premolars. In this respect they are in marked contrast to the Carnivores, in 

 which, as we have seen, there is a great tendency to a reduction 

 in the number of the molar teeth, only one living member of 

 the order (the long-eared fox) having the typical three molar 

 teeth in the upper jaw. On the other hand, among the more 

 specialised representatives of the order, there is a decided tend- 

 ency to the reduction, either in size or number, of the front 

 teeth ; the tusks being very frequently small or absent, while 

 the whole of the incisor teeth, and sometimes the canines also, 

 in the upper jaw, and more rarely both incisors and canines 

 in the upper and lower jaws, may be wanting. All the earlier 

 Ungulates, as well as the modern pigs, have, however, well- 

 developed tusks, as well as the full number of front teeth; 

 and it is thus apparent that in this respect also the result of specialisation has 

 been the reverse of that in the Carnivores, where the tusks have obtained 

 extreme development, and the full typical number of incisor teeth is very 

 generally retained. In both cases these distinctions are due to the differ- 

 ence in the nature of the food and habits of the two groups of animals. In 

 addition to these characters of their feet and teeth, the Ungulates of 

 the present day are characterised by the total absence of collar - bones or 

 clavicles in the adult condition, although traces of these may occur in the foetal 

 state. 



RIGHT UPPER MOLAR 

 TOOTH OF THE EX- 

 TINCT MERYCOPO- 

 TAMUS, VIEWED 

 FROM ABOVE. 



