xxxi. 7 CAMELS 753 



tubular organ with glandular lining. These differences suggest that 

 the ruminating habit may have been evolved separately in camels and 

 Ruminantia (Bohlken, i960). 



In the head there are many features of similarity to the Pecora. 

 Cropping is by means of procumbent lower incisors, working against 

 specialized premaxillary gums ; but an upper incisor and canine are still 

 present (three incisors in the young). The molars have a typical 

 selenodont pattern, and the structure of the skull shows the develop- 

 ments so commonly seen as a result of herbivorous life, such as closure 

 of the post-orbital bar. The lips and tongue are tough and able to 

 chew spiny desert plants. A peculiar feature of camels is that the red 

 blood corpuscles are oval, as in no other mammal. The placenta is of 

 a diffuse (non-cotyledonary) syndesmochorial type. 



There is no doubt that many of these features have developed 

 independently in camels and Pecora; the Eocene camel-ancestors did 

 not show them. At the stage of *Protylopus the camels were small and 

 had short limbs, with separate radius and ulna and four digits in the 

 manus. Throughout the Oligocene many primitive features still 

 remained. *Poebrotherium was about 3 ft high, with a complete denti- 

 tion, and orbit only partly closed behind. However, the lateral toes 

 had been lost and the digits began to diverge distally, though probably 

 still carrying hoofs. The remaining increase of size, and the develop- 

 ment of other special camel features can be traced slowly through such 

 types as *Procamelus of the Miocene and Pliocene. During the dry 

 Miocene times the type was very successful in North America and 

 developed various lines, such as *Alticamelus, the giraffe- camels, with 

 long necks. 



7. Ruminants 



The most successful modern artiodactyls, the deer (Cervidae) and 

 cattle, sheep, and antelopes (Bovidae), have flourished only since the 

 Miocene and are thus the most recent ungulate group, largely re- 

 placing the tylopods, oreodonts, perissodactyls, and still earlier prot- 

 ungulate and paenungulate types. They have always been mainly an 

 Old World group and this remains their headquarters, though some 

 have reached other parts of the world. The early ancestors of the 

 ruminants can be traced to Oligocene and late Eocene forms very like 

 the early camels, oreodonts, and other primitive artiodactyls; the 

 modern chevrotains (Tragulns) retain some of the features of this 

 early stage. 



The characteristic features of the ruminants are the full develop- 



