THE PALEONTOLOGIC RECORD 293 
Secondly, the change of axis is not in the ancestral direction, the 
excessive weak condition of the jaws being again an embryonic adapta- 
tion and not an ancestral one. 
Lastly, in the development of cancellous tissue is a condition which 
more nearly approximates the phylogenetic development, but here even 
the use of young features is deceptive, for it is seldom that this cellu- 
lar bone is developed in the immediate ancestor but is rather found in 
several genera back, being usually an accompaniment of the develop- 
ment of the heavy facial portion of the skull. So much for form. 
Turning to the dentition. The milk set of the pig and those of the 
adult are drawn side by side, and it is seen that while the front teeth 
of the young approximate those of the adult, the comparison is between 
the complicated premolar and molar sets. Briefly, of the four pre- 
molars, if all present, in the young (and often but three are developed ) 
the two in front resemble the premolars to succeed them in the perma- 
nent set, while the two rear milk premolars resemble the permanent 
molars, the last milk premolar being especially like the last molar. 
This granted, the interest centers around whether the pattern of the 
milk teeth is such as to indicate the ancestry. A glance at the pig and 
its young will show that while the detail is not exactly the same in young 
and old, yet they are so alike that no one would identify a single milk 
molar as Hyotheriwm or any other suine genus, but would have to put 
it in the genus Sus. Taking other cases among the Ungulata, the his- 
tory of the naming of the Miocene genera of horses gives a good ex- 
ample. There are, according to Gidley, four genera, H yohippus, Para- 
hippus, Merychippus and Protohippus ; of these, three were founded on 
young teeth, 1. ¢., the first three named. When it was recognized that 
they were young teeth, they were by Cope assigned to Protohippus, but 
when the adult teeth were found it was clear that the distinctive features 
of these young teeth were the distinctive features of the adult. For the 
genus Merychippus there is a difference in that the young teeth are not 
cemented, while the adult are. That is ancestral. In analyzing the 
descriptions of several genera of horses usually some feature can be 
found in the milk tooth which is ancestral. 
In the Carnivora there is the carnassial tooth which is specialized ; 
in the upper jaw it is the third milk premolar and the fourth in the 
adult; in the lower jaw it is the fourth milk premolar, and the first 
molar of the adult. Thus it is clear that it is a different dental follicle 
which forms the young and the adult carnassial. In the case of the dog 
the permanent and milk carnassials are approximately alike, but in the 
case of the cat the inner lobe or protocone occupies a very different 
place in the young from that of the adult, a position characteristic of 
none of the Felide and suggests some of the apparently unrelated 
Creodonts. 
