EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 455 



Comparative measurements, in millimeters, of deciduous and permanent dentition of species of Menodus and Brontops 



SUMMARY OF THE REPLACEMENT OF THE TEETH IN 

 OLIGOCENE TITANOTHERES 



The following is a summary of the order of succes- 

 sion of the upper and lower teeth as observed in the 

 six juvenile stages represented in Plates XXIII and 

 XXIV and in Figures 384 and 385. 



1. The three deciduous incisors (di-^, f, f) have 

 the same relative position as the permanent incisors 

 (Ml i> f) ^ Teleodus avus. They were shed very 

 early. 



2. The deciduous canines, known only from their 

 alveoli in one specimen (PL XXIV, A), were shed 

 perhaps even earlier than the deciduous incisors 

 (PI. XXIII, XXIV). 



3. No evidence of deciduous predecessors of p' and 

 Pi has been observed either in Eocene or in Oligocene 

 titanotheres, and in these, as in other ungulates, there 

 were probably only three and not four deciduous pre- 

 molars. 



4. The permanent p^ came into place soon after 

 the deciduous dpf , f and functioned with the decid- 

 uous series. 



5. The first true molars (m^) came into place be- 

 fore the deciduous premolars had been replaced. At 

 later periods mf and mf came into place successively, 

 so that in old animals m-^ is greatly worn, whereas 

 mf is but little worn. 



6. The fourth premolar (p|) follows the general 

 mammalian rule of coming in late. 



In the Oligocene titanotheres, as in many other 

 mammals, the second, third, and fourth deciduous 

 premolars (dpf, f, f) are much more molariform 

 than the permanent premolars (pf, |, |) which re- 

 place them. Thus molarization of the deciduous 

 premolars is observed even in the middle and lower 

 Eocene titanotheres {LamMotherium, Palaeosyops) 

 and is equally characteristic of the Oligocene titano- 

 theres. In Menodus giganteus (Am. Mus. 497, PI. 

 XXV, A) the third deciduous premolars (dpf) are like 

 molars, dp' and dp* having prominent mesostyles and 

 large, distinct tetartocones. 



101959— 29— VOL 1 32 



Figure 387. — Stages of wear in the adult upper grinding teeth 

 of Oligocene titanotheres 



The following specimens, all drawn to the same length, show the progressive degrees 

 of wear from the young adult (X) to the very aged (XV) ontogenetic stage of the 

 dentition: X, Allops crassieornis, Nat. Mus. 4289 (type); XII, Brontops roiustus, 

 (type), Yale Mus. 12048; XIII, Menodus giganteus. Am. Mus. 505; XIV, Allops 

 serotinus. Am. Mus. 620; XV, Brontops brachycephaJus, Nat. Mus. 4947. (See 

 p. 456 ) 



