Mammalia: Classification 751 



Fig. 563. Australian dog, Canis dingo. (Courtesy, Museum of Comparative Zool- 

 ogy, Harvard University.) 



ungulate features of several sharply demarked groups of modern 

 mammals are an instance of convergent specialization along lines of 

 independent origin. 



The following four Orders (13-16) include "ungulate" mammals. 

 The hoof is a massive horny structure developed, in place of a claw, 

 on the distal phalanx of a digit, providing a broad flat surface of 

 contact with the ground. The most significant difference between 

 Perissodactyla and Artiodactyla is the structure of the feet. The 

 members of both Orders are unguligrade (see p. 583), and more or 

 less reduction in number of digits occurs, but the plan of symmetry 

 involved in the reduction is radically different in the two Orders (Fig. 

 564). The commonly used terms "odd-toed" and "even-toed" must 

 not be taken literally. The "odd-toed" tapir has four digits on the 

 manus, and at least the primitive artiodactyls, "even-toed," had all 

 five digits. The perissodactyl plan of symmetry, mesaxonic, tends 

 toward an odd-toed foot, and the artiodactyl plan, paraxonic, tends 

 toward an even-toed foot. 



Order 13: PERISSODACTYLA. "Odd-toed" or mesaxonic. 



The axis of symmetry of the foot is the third digit (Fig. 564). The 

 second and fourth digits are equally and symmetrically developed, and 

 the first and fifth, if present, are likewise equally developed. The third 

 digit is strongest and, in cases of extreme redaction, is the only func- 

 tional digit. During locomotion only the tips of the functional digits 

 touch the ground (Fig. 450). 



