GENERAL CHARACTERISTICS. 155 
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 
erescent -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. 

Tt should also be mentioned that whereas in 
Gumivoresthe upper molar teeth are'gener- 9 4° Dee cua INNER AND OGREE 
SEED ] N} AND OUTER 
ally of the primitive triangular type, in all SIDES. 
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 at 
the whole of the incisor teeth, and sometimes the canines also, 4 picur upper MoLAR 

in the upper jaw, and more rarely both incisors and canines TOOTH OF THE EX- 
in th m ¢€ al ] r — - ~~ - = b me ti mil tl Ne li 5 TINCT MERYCOPO- 
1 e upper and lower jaws, may be wanting. ye earlier Sane, GOES 
Ungulates, as well as the modern pigs, have, however, well- FROM ABOVE. 
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. 
