52 THE PALEONTOLOGIC RECORD 



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 Hyotherium 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, Hyohippus, Para- 

 hippus, Merychippus and Protohippus; of these, three were founded on 

 young teeth, i. e., 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 Felidae and suggests some of the apparently unrelated 

 Creodonts. 



