242 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



(diastema) from the lateral incisor in front of it, and from the premolar behind 



it. The premolar teeth are three in number, and increase in size from before 



backwards ; the first is very small with a relatively immense, sharp, central 



cusp, supplemented by minute anterior and posterior cusps. The second 



premolar tooth is larger, the central cusp still immense, and an internal cusp 



is suggested on the surface of the cingulum (a band which surrounds the base 



of the tooth-crown) ; in the third premolar tooth this internal cusp is still 



more marked. The molar teeth are tricuspid (or tritubercular, cf. p. 283) and 



the cingulum is distinct, forming internally a prominent band which skirts 



the inner of the three cusps ; the other two cusps are external (cf. Fig. 161). 



The third and last molar tooth is much smaller than the second, and as this 



is usually smaller than the first, the series diminishes backwards. In the 



mandible (Fig. 162) the formula (v. supra) requires some elucidation, for the 



canine tooth (judged by the formida) is apparently incisor-like in form, 



and it is also procumbent, or directed horizontally forwards like the 



definite incisor teeth. Moreover, the first premolar tooth resembles a 



canine tooth. The question of the identification of these teeth is still in 



doubt ; Forsyth-Major (Geological Magazine, 1900) has adopted the view 



which regards the canine-like tooth as the genuine canine, and the outer 



incisor-like tooth as an incisor, thus providing the mandible with a dental 



formula i, 3 ; c, 1 ; pm, 2 ; m, 3 ; different, that is, from the maxillary 



formula i, 2 ; c, 1 ; pm, 3 ; m, 3. But it is to be noticed that against this view 



may be urged the fact that the canine-like premolar tooth although admittedly 



canine in form, yet closes (in the bite) posteriorly, and not anteriorly, to the 



upper undoubted canine tooth. Yet this posterior position of a lower canine 



tooth is a phenomenon of extreme rarity, and though Forsyth- Major defends 



his position by appealing to the progressive shortening of the mandible, which 



no doubt has occurred, still in view of the mutability of dental 



forms, it seems more consistent to adopt the opposite view of 



regarding the canine tooth of the mandible as incisiform, and 



the first premolar tooth as caniniform 1 . 



The lower teeth. All the mandibular teeth are flattened 



from side to side, the premolars bear one relatively immense 



cusp and three minute subsidiary cusps (cf. Fig. 171), and 



the middle premolar tooth is the smallest of the three. The 



molar teeth are quadricuspid, two cusps being external and 



larger than the two remaining and internally situated cusps, 



and these teeth diminish in size backwards. Such diminution 4 



backwards suggests that the jaws are undergoing a process ^. ' l ^' „ , u ' 



& ° ... . Diagram of the 



of reduction in length which is in operation at the posterior fi rs t left lower 



extremity. Forsyth-Major has pointed out that, in addition premolar tooth 



to this, retraction of the mandible as a whole has occurred. 



1 Flower and Lydekker, Mammals, p. 683, suggest that the canine tooth is 

 absent altogether. 



