224 
OX THE STKUCTUEE AND CLASSIFICATION 
distinguish the mammals of this period, so far as known, very clearly from the 
Mulfituberculaia and P)-otoclonta. 
In this, as in the former divisions, the molar pattern forms an advantageous 
starting ^wint for classification. The entire dentition is in fact distinctly, although 
not highly, specialized ; in carefully studying the details of the numerous members of 
this group we invariably find a certain form of premolar, canine and incisor accom- 
panying a certain molar pattern and an equally fixed relation existing between the 
dentition and the characters of the mandible. In some cases where the molars are 
apparently rather diverse, the structure of the remaining teeth and of the mandible 
brings us back to the conclusion that there is some affiliation. As the genera fall 
into smaller groups we at once observe that these groups shoAv a more or less clearly 
marked specialization for a certain kind of diet, which is usually manifested most 
clearly in one genus, which we may therefore speak of as typical of the group. The 
allied genera diverge more or less widely from this type, which, it must be understood, is 
not selected as the most primitive or the most central but as showing the most clearly 
defined functional adaptation. While some genera are thus functionally typical others 
are traimtioruil, that is, they show a divergence from the central type towards a different 
kind of adaptation. Still other genera are isolated ; they do not approach other known 
tyixjs but stand apart by themselves, either because few specimens have been found and 
wc are less familiar with their structure, or their molar pattern does not conform with 
that of any other known genus, or represent a distinct type. Our first object then is, 
where i)o.ssible, to group the genera into families; secondly, to unite these families 
into what we may call sul)-groups, indicating their general adaptation to a certain 
diet. Ihe sub-groups naturally have less permanent taxonomic value than the 
families, and still less than the genera. The families are thus grouped where they 
seem to show evidence of being in early stages of differentiation along certain lines of 
ftmctional adaptation. These lines are not sharply defined, but by a comparison of 
the typical forms of each of these sub-groups with the most nearly allied recent genera 
we may divide them into carnivorous, omnivorous, herbivorous and insectivorous series, 
as indicating, in most cases, an initial rather than an advanced stage of specialization. 
♦ 1 e! ; "lost part, clearly distinguished from each other, but 
the is the only fully defined family, in which numerous genera, with 
ZuM n ZZT 'r“‘‘ 'i^UHonofboth jaws known, ase 
VO ™ •» boundaries when 
"r f” example, that the 
I'nconodonUdiv will embrace a larger number of genera 
