THE PERISSODACTTLA. 295 



a. The Equidce, or Horses and Asses, have one toe on 

 each foot — the third — much longer and larger than the rest. 

 The latter are represented only by their metacarpal or meta- 

 tarsal bones, the inner and outer toes being absent, or repre- 

 sented by mere ossicles (as rudiments of their metacarpals or 

 metatarsals) in all existing JEquidce. But, in the extinct Sip- 

 2?arion, the second and fourth digits were complete, though 

 small and like dew-claws ; while the miocene Atic/iitherium, 

 which most nearly approaches the Palmotheridm, has the lat- 

 eral toes much larger, and taking their share in supporting 

 the weight of the body. 



The dental formula is i. |^' c. \^^ p.m. ~ m. 3-4^. The 

 tooth here counted as the first premolar may be a milk-tooth, 

 as it appears to have neither predecessor nor successor, and 

 soon disappears. 



The molar teeth present an outer wall, which is bicrescen- 

 tic in transverse section ; and two inner ridges, which are 

 curved more or less inward and backward, and correspond re- 

 spectively with the anterior and the posterior crescents of the 

 outer wall. The valleys may be more or less completely filled 

 up with cement, which also coats the tooth. The incisors are 

 similar in form in each jaw, and in Equus and Hlpparion their 

 crowns present a wide and deep median cavitj', formed by a 

 fold of the enamel. 



These are the distinctive characters of the JEquidce. It 

 may be useful to add some special details respecting the anat- 

 omy of the Horse as a familiar example of the perissodactyle 

 group. 



The Horse has seven cervical vertebrae, twenty-four dorso- 

 lumbar (eighteen or nineteen of which are dorsal), five sacral, 

 and about seventeen caudal vertebrse. The atlas has very 



Fia. 95.— A cerrlcal vertetra of a Horse.— 1. The rudimentary spine. 2, 3. Tie pre- and 

 posi^zygapopLyses. 5. The convex anterior face of the centrum. 9. Its concave pos- 

 terior face. 6, T. The transverse processes and rudimentary ribs. 



