30 GENERAL ANATOMICAL CHARACTERS 
of the herbivorous type. The Marsupialia exhibit examples of 
all forms, except the purely piscivorous. Other orders, more 
restricted in number or in habits, as the Proboscidea and Cetacea, 
naturally do not show so great a variety in the dental structure of 
their members. 
Taxonomy.—In considering the taxonomic value to be assigned to 
the modifications of teeth of mammals, two principles, often 
opposed to each other, which have been at work in producing these 
modifications, must be held in view:—(1) the type, or ancestral 
form, as we generally now call it, characteristic of each group, 
which in most mammals is itself derived from the still more 
generalised type described above; and (2) variations which have 
taken place from this type, generally in accordance with special 
functions which the teeth are called upon to fulfil in particular 
cases. These variations are sometimes so great as completely to 
mask the primitive type, and in this way the dentition of many 
animals of widely different origin has come to present a remarkable 
superficial resemblance, as in the case of the Wombat (a Marsupial), 
the Aye-Aye (a Lemur), and the Rodents, or as in the case of the 
Thylacine and the Dog. In all these examples indications may 
generally be found of the true nature of the case by examining the 
earlier conditions of dentition; for the characters of the milk- 
teeth or the presence of rudimentary or deciduous members of the 
permanent set will generally indicate the route by which the 
specialised dentition of the adult has been derived. It is perhaps 
owing to the importance of the dental armature to the well-being 
of the animal in procuring its sustenance, and preserving its life 
from the attacks of enemies, that great changes appear to have 
taken place so readily, and with such comparative rapidity, in the 
forms of these organs—changes often accompanied with but little 
modification in the general structure of the animal. Of this 
proposition the Aye-Aye (Chiromys) among Lemurs, the Walrus 
among Seals, and the Narwhal among Dolphins form striking 
examples ; since in all these forms the superficial characters of their 
dentition would entirely separate them from the animals with which 
all other evidence (even including the mode of development of their 
teeth) proves their close affinity. 
Trituberculism.—Recent researches, and more especially those of 
Professors Cope and Osborn, tend to show that almost all of the 
extremely different forms of tooth-structure found among Mammals 
may be traced to one common type, in which the crown of each 
tooth carried three cusps, and hence termed the fritubercular type ; 
these three cusps being arranged in a triangle, with the apex 
directed inwardly in the upper teeth (Fig. 4, 6), and outwardly in 
the lower ones (Fig. 4, 7). It is further probable that this 
tritubercular type was itself derived from a type of dentition in 
