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 Eodents, 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 (Chiromyx) 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 Avhich the crown of each 

 tooth carried three cusps, and hence termed the tritubercular type ; 

 these three cusps being arranged in a triangle, with the apex 

 directed inwardly in the upper teeth (Fig. 4, e), 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 



