THE TEETH 267 



reduction is proved by the fact that in extinct forms replace- 

 ment took place in more of the teeth. There is another point 

 of interest in the teeth of the Marsupials, which refers to the 

 fact that they are the only mammals in which more than three 

 incisors are found on each side. The probable explanation 

 is that in this region of the mouth, the teeth of one family are 

 not replaced by the teeth of the other, but that both families 

 of teeth are erupted together, the members of the two families 

 intercalated as in the crocodile. Behind the canine, however, 

 the families of teeth replace one another as in other mammals. 

 The marsupials, then, are intermediate between the reptiles 

 (simultaneous presence of teeth of both families all over the 

 jaw with complete intercalation) and the higher mammals (no 

 intercalation of teeth of the two families). 



The primitive shape of the molar teeth in the mammal is 

 three-cusped or tritubercular in the upper jaw, while those of 

 the lower jaw have three cusps and a posterior " heel " or 

 talonid, and are called tuberculo-sectorial. The three cusps 

 of the upper teeth form a triangle or " trigon," with the apex 

 pointing inwards ; the three cusps of the lower teeth form a 

 " trigonid," with the apex pointing outwards. They are so 

 arranged by this means that the teeth of the upper and those 

 of the lower jaw fit into and work against one another. This 

 type of molar was evolved from the primitive reptilian type 

 in which each tooth had but one cusp. The original cusp 

 is represented by the outer cusp of the trigonid in the lower 

 molars, while in the upper molars the original cusp has been 

 split into two and is represented by the two lateral cusps of 

 the trigon. The remaining cusps and the talonid were 

 subsequently developed in relation to the " fit " of the teeth 

 on one another. The number and arrangement of the cusps 

 may be much modified in the different groups, but the primitive 



of selenodont teeth. G, crown of an unworn, H, crown of a worn lower 

 molar of a tapir, showing the transverse ridges joining the cusps, character- 

 istic of lophodont teeth. I, inner side view of a tritubercular (upper) 

 molar, in relation to J, a tuberculo-sectorial (lower) molar ; t, talonid. K, 

 diagram of the relative positions of the cusps of tritubercular molars (dotted 

 lines) of the upper jaw, and tuberculo-sectorial molars (full lines) of the 

 lower jaw ; an, anterior side ; ou, outer side. L, simple peg-like tooth of 

 a reptile. 



