THE HOLLOW-HORNED RUMINANTS 



A LEFT UPPER MOLAR TOOTH OF THE 

 NYLGHAU, SEEN FROM THE INNER 

 AND OUTER SIDES. 



cheek-teeth have low columns, or ridges, so that the bases of the-intervening valleys 

 can be distinctly seen at all 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 di- 

 vided. It should also be mentioned that 

 whereas in Carnivores the upper molar teeth 

 are generally 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 specialized kinds; and to the same 

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

 ber 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 or- 

 der (the long-eared fox) having the typical three molar teeth in 

 the upper jaw. On the other hand, among the more specialized 

 representatives of the order, there is a decided tendency to the 

 reduction, either in size or number, of the front teeth; the tusks 

 being 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, maybe 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 specialization has been the reverse of that in the Carni- 

 vores, 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 difference in the nature of the food and habits of the two 

 groups of animals. In addition to these characteristics of their feet and teeth, 

 the Ungulates of the present day are characterized by the total absence of collar 



A RIGHT UPPER MO- 

 LAR TOOTH OF THE 

 EXTINCT MERYCO- 

 POTAMUS, VIEWED 

 FROM ABOVE. 



