246 COMPARATIVE ANATOMY 



teeth, or grinders, can. as a general rule be distinguished. The 

 teeth which replace the milk -grinders are distinguished a&premolars 

 from the molars, which are situated further back in the jaw and 

 have no predecessors. 1 



All the teeth are imbedded in well-developed alveoli of the 

 jaw-bones, the upper incisors being situated in the premaxilloe, the 

 upper canines and cheek-teeth in the maxilla?, and the lower teeth 

 in the mandible (dentary). The canine, which corresponds to a 

 specially differentiated premolar, and is most characteristically 

 developed in Carniyora, lies in a- more or less continuous series 

 with the incisors. The premolars follow behind the canine, the 

 space usually present between them being called the diastema, and 

 then come the molars. The primary arrangement of the teeth is 

 such that there is an alternation between those of the upper and 

 lower jaw : thus the teeth in one jaw do not usually correspond in 

 position with those of the other, but with the interspaces between 

 them. 



In some cases the enamel-organ persists in all the teeth, 

 which then continue to grow throughout life (e.g., Lepus) ; in. 

 others this is true of the incisors only (e.g., numerous Rodents, 

 Elephant) ; but more usually growth ceases after a certain time, 

 and the teeth then form de finite fangs or roots, each perforated by a 

 small canal communicating with the reduced pulp-cavity. 



The incisors are usually chisel-shaped, while the canines, in 

 those cases where they are most characteristically developed 

 (Carnivora), possess a pointed, conical form, and are more or 

 less curved. The cheek-teeth either possess sharp, cutting crowns 

 (e.g., Carnivora), or the crowns are broad and more or less flat and 

 tuberculated, and adapted for grinding the food. In the latter 

 case the relations of the enamel, dentine, and cement are such as 

 to produce an uneven surface with wear, showing a characteristic 

 pattern in the different groups (Figs. 196200). 



The relations and number of the tubercles which may be conical (e.g., Pig) 

 or cresceiitic (e.g., Horse, Ruminants, Fig. 199), as well as the form of the 

 teeth in general, is of great importance in elucidating the ancestral history 

 of the Mammalia, and attempts have been made to trace the evolution of 

 the various forms of molar met with in the Class. According to one view 

 the tuberculated molar has arisen by the gradual modification of a single 

 conical tooth, which has produced lateral outgrowths or buds. Thus taking the 

 simple conical form such as exists in toothed Whales as the most primitive 

 form of mammalian tooth, we find that certain extinct Mammals (e. g. , Trico- 

 nodon) possessed teeth with a main cone and two lateral cusps. It has been 

 supposed that the more complicated forms have been derived from this 

 triconodont tooth firstly by a rotation of the lateral cusps outwards in the 

 upper, and inwards in the lower tooth, thus forming a tritubercular tooth, 

 with three cusps arranged in a triangle ; and secondly by the addition of other 

 cusps, the first to appear being the posterior heel or talon. 



Another hypothesis is that the mammalian cheek-teeth were primarily 



1 It must, however, be remembered that in some cases the so-called pre- 

 molars have no predecessors (see p. 249). 



