392 VERTEBRATE ZOOLOGY 



nants. It includes oxen, sheep, goats (Fig. 199, F) and antelopes. 

 The most prominent distinguishing character of the group is the 

 horns, which are hollow and composed of chitin, and are usually 

 present in both sexes. A large number of the Bovidse have been 

 domesticated, and from the human standpoint are the most impor- 

 tant of all animals. The members of the group are so familiar that 

 no description of the different species is necessary. 



ORDER 11. PERISSODACTYLA (ODD-TOED UNGULATES). In this 

 group the middle digit of both fore and hind feet is preeminent and 

 carries most of the weight. The axis of the limb passes through the 

 third digit. The teeth of the odd-toed ungulates are usually lophodont, 

 a type characterized by the presence of enamel ridges running back 

 and forth across the grinding surface. The present-day perissodactyls 

 are grouped into three families: Equidae, Tapiridse, and Rhinocer- 

 otidae. 



Family 1. Equidce (horses, asses, and zebras). The members of the 

 horse family (Fig. 200, A) are characterized by the possession of but 

 a single functional toe, the third toe, on each foot. The second and 

 fourth toes are represented by vestigial remnants, called " splint 

 bones." The molar teeth are highly complex in structure and wear 

 down through most of the life of the individual, so that the age of 

 any specimen may be arrived at by the amount of wear upon the 

 teeth. All of the modern Equidae are placed in the single genus 

 Equus. Perhaps the most convincing record of the ancestry of any 

 vertebrate group is that of the horse. With respect to toes, teeth, 

 and general form, the gradual perfection of the present highly spe- 

 cialized cursorial type may be traced back through an unbroken 

 line of ancestors to a very generalized ungulate type with four func- 

 tional toes, generalized teeth, and comparatively small size. The 

 horse has played and is still playing an extremely important role in 

 the progress of human civilization. Next to cattle and sheep the 

 horse has been the most important domesticated animal; but if 

 present tendencies furnish a reliable criterion of the future, the horse 

 is likely to be displaced by the motor-driven vehicle. 



Family 2. Tapiridce (Tapirs) The tapirs (Fig. 200, B) are the 

 most generalized of modern odd-toed ungulates. They are charac- 

 terized by moderate size and by a short proboscis produced by 

 elongation of nose and upper lip. The dentition is more generalized 

 than that of the horses, there being forty-two teeth, a number very 



