458 EVOLUTIONARY MORPHOLOGY 



on the molars, fusion of ulna with radius and tibia with fibula, 

 specialisation of the wrist and ankle joints into articulations 

 allowing movement in only one plane, enlargement of the 3rd 

 digit in hands and feet, and reduction of all the other digits 

 until their disappearance. 



The evolution of the camels from the Eocene Protylopus 

 through Poebrotherium (Oligocene), Procamelus (Miocene) to 

 the present day is likewise a history of gradual increase in size, 

 increase in length of the teeth and development of selenodont 

 ridges on the molars, reduction of the upper incisors, enlarge- 

 ment of the 3rd and 4th digits in hands and feet with suppres- 

 sion of the remainder, and fusion of the 3rd and 4th meta- 

 carpals and metatarsals. 



So far as is known the history of the elephants starts with 

 the Eocene Moeritherium, of about the size of a pig, and 

 with the primitive dental formula of i|, c^, pf, nig. Its 

 ridged (lophodont) molars had only two ridges. In the upper 

 Eocene, Palseomastodon was larger, and had a not inconsider- 

 able trunk. The canines and all the incisors except one pair 

 in each jaw had disappeared, and the molars had three ridges. 

 Tetrabelodon from the Pliocene was still larger and its incisors 

 were elongated into tusks, with persistent pulps. The molars 

 had as many as six ridges and were so large that there was 

 not room in the jaws for more than two teeth in each jaw on 

 each side. Furthermore, instead of being replaced from 

 beneath as in ordinary mammals, they were replaced from 

 behind, the new tooth pushing the old one out forwards in 

 front of it. It is worth noticing that although these animals 

 grew large and tall, their necks were very short, and it is only 

 by means of the long trunk that they were able to reach down 

 to the ground for eating and drinking. The next step, shown 

 by the Pliocene Mastodon, was accomplished by a shortening 

 of the lower jaw and the loss of the lower incisor- tusks. Lastly 

 in Elephas the grooves between the ridges on the molars become 

 filled with cement. The ridges may be a dozen in number, 

 and the maximum number of molars on each side in each jaw 

 in use at one time is one and a half. 



The history of the Primates is reserved for the next chapter. 



