XI PALEONTOLOGY AND EVOLUTION 361 



nished with very strong reasons for looking for a 

 no less complete verification of the expectation 

 that the three-toed Plagiolophus-like " avus " of the 

 horse must have had a five-toed " atavus " at some 

 earlier period. 



No such five-toed " atavus," however, has yet 

 made its appearance among the few middle and 

 older Eocene Mammalia which are known. 



Another series of closely affiliated forms, though 

 the evidence they afford is perhaps less complete 

 than that of the Equine series, is presented to 

 us by the Dichobune of the Eocene epoch, the 

 Cainotherium of the Miocene, and the Tragulidce, 

 or so-called " Musk-deer,'' of the present day. 



The TragulidcB have no incisors in the upper 

 jaw, and only six grinding-teeth on each side of 

 each jaw ; while the canine is moved up to the 

 outer incisor, and there is a diastema in the lower 

 jaw. There are four complete toes on the hind 

 foot, but the middle metatarsals usually become, 

 sooner or later, ankylosed into a cannon bone. 

 The navicular and the cuboid unite, and the 

 distal end of the fibula is ankylosed with the 

 tibia. 



In Cainotherium and Dichobune the upper 

 incisors are fully developed. There are seven 

 grinders ; the teeth form a continuous series with- 

 out a diastema. The metatarsals, the navicular 

 and cuboid, and the distal end of. the fibula, 

 remain free. In the Cainotherium , also, the second 



