INTRODUCTION 



17 



tubercular crowns are used for the former purpose, whereas those with broad 

 crowns provided with pointed cusps serve the latter purpose. Insectivores 

 and carnivores have, therefore, cutting, multi-tubercular (secodont) crowns on 

 the cheek-teeth which are very eflfective, and function as shears. The upper 

 teeth override the lower, and close outside them. Animals of omnivorous 

 diet have a bunodont dentition ; that is, the low crowns of the cheek-teeth are 

 enlarged and provided with conical cusps. The articulation of the lower jaw, 

 as in the case of insectivores and carnivores, is vertical (orthal), and the 

 condyle is transversely directed. 



The cheek-teeth of herbivorous mammals reach the highest stage of 

 specialisation. Here also we find the common tendency to enlarge these teeth 



tal? 



Fig. 18. 



Hypothetical development of the tritubercular tooth pattern. A^ Inner aspect of upper and lower molars. 

 B, Upper and lower molars superimposed, pi', protocone ; 'pa,, paracone ; ^rd, protoconid ; pad, paraconid ; 

 med, metaconid ; e,nd, entoconid ; tald, talonid. (After Gregory.) 



JJ7 





FiG. 19. 



A, Dryolestes. Upper Jurassic. Lower molar still tritubercular but without hypoconid ; upper molar 

 tritubercular with parastyle. B, Triisodon. Lower Eocene. Lower molars tuberculo-sectorial. Both tooth 

 rows superimposed, pr, pa, prd, pad, nied, end, as in Fig. 18 ; me, inetacone ; ps, parastyle ; ml, metaconule ; 

 hyd, hypoconid. (After Gregory.) 



in length and breadth, and to provide them with cusps for the grinding of 

 the food. The conical cusps of the original bunodont tooth become V-shaped, 

 in that two converging outer ridges are formed. Either by the pronounced 

 development of the branches of the V-shaped ridges, or by the compression of 

 their extremities, or again by the union of the two cusps by a straight or 

 curved transverse ridge, a complicated lophodont crown, with many crests, 

 ensues. The motion of the lower jaw has an especial influence on the 

 formation and direction of these ridges. If the mandible moves extero- 

 interiorly (ectal) in the chewing process, as is the case with most herbivorous 

 ungulates, or intero- exteriorly (ental), the V-shaped crests are arranged 

 longitudinally one behind the other, and the borders of the cusps as well as the 

 transverse ridges are placed at right angles to these. Very frequently the 

 apices of the V-shaped ridges are worn down to form crescents (selenodont 

 ridges). If the lower jaw moves in an antero-posterior plane (proal mastication), 

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