752 



Comparative Morphology of Chbrdates 



PERISSODACTYL 



ARTIODACTYL 



Fig. 564. {Left) Feet of horses. Manus at top, pes at bottom. (A) Eohippiis, a 

 primitive Lower Eocene perissodactyl with four toes in front, three behind. (B) 

 Miohippus, an Oligocene three-toed horse. (C) Merychippm, a late Miocene horse 

 with reduced lateral toes. (D) Eqmis. (A, after Cope; B and C, after Osborn. 

 Courtesy, Romer: "Vertebrate Paleontology," University of Chicago Press.) 



(Right) Feet of pig. Manus at right, pes at left. 



The dentition is of the herbivorous type: incisors adapted for cut- 

 ting, canines small, cheek-teeth flat-topped and ridged for crushing 



3 1 

 and grinding; formula for horse, i ^> c y 



4 3 

 p -> m k- The stomach is 



4 -3 



simple and there is no gallbladder. 



There are three Families of modern perissodactyls. 



Equidae: Only the third digit (in modern Equidae) functionally 

 developed; vestiges of the second and fourth form the pair of ""splint 

 bones" attached lateralis to the elongated third metacarpal or meta- 

 tarsal (Fig. 561). Horse, ass, zebra (Equus). 



Tapiridae: Four digits on manus (pollex lacking), three on pes. 

 Snout elongated into a short proboscis. Tapir of the Malayan region 

 and South and Central America (Fig. 565). 



