LIFE IN TERTIARY TIMES 305 



transformed into a kind of beak. Dinoceras, lacking upper 

 incisors, was armed with three pairs of horns analogous 

 to those of the Rhinoceros, whose upper incisors have also 

 disappeared. These teeth were also very small or altogether 

 missing in Titanotherium, the most pronouncedly horned 

 form of Perissodactyl ; but the reduction of the number of 

 teeth begins in their series with the appearance of horns. 

 The canines are already weak in Hyracodon of the White 

 River Oligocene, and disappear from the upper jaw of the 

 first Rhinoceroses, which have no horns (Aceratherium), and 

 retain only two pair of incisors in the upper and one pair in 

 the lower jaw. They migrated to the former continent and 

 arrived in India during the Upper Miocene, and disappeared 

 in the Pliocene. During this Period the American Rhinoceros 

 (Diceratherium) acquired symmetrical horns ; the true 

 Rhinoceros has only one median horn or two placed one behind 

 the other. These already existed in Europe during the Middle 

 Miocene (Sansans), where their most highly modified repre- 

 sentatives presented neither incisors nor canines. This is also 

 the case in the African Rhinoceros (A telodus) , in the Rhinoceros of 

 Pikermi (AteloduspacJiygnathus) , and the Rhinoceros Tichorhinus 

 (Calodonta), the contemporary of man. The molars themselves 

 were reduced to five, and had bands of enamel extraordinarily 

 folded in the gigantic El as mother ium of Siberia, which had 

 a head a metre long, and an enormous horn on its forehead. 



From what we have seen above it follows that it is no more 

 possible to affirm in the lineage of the Rhinoceros than in that 

 of the Ruminants that the reduction in the number and dimen- 

 sions of the teeth present, particularly in the upper jaw, 

 can be due to the development of horns. Nevertheless, we are 

 dealing with such a remarkable coincidence that we have 

 the right to ask whether some fundamental relationship does 

 not exist between these two phenomena, connected with 

 some competition for the valuable lime salts to which both 

 teeth and horns must have recourse in their development. 



The oldest horned Ruminants date back to the Oligocene 

 Period, and from that time forward were liberally provided in 

 that respect. These were species of Protoceras of the White River 

 in America. They had then four well-developed digits in front, 

 but two only and a lateral splint behind, large canines, and 

 ten pairs of horns in the male, reduced to two in the female. 



