MAMMALIA. ARTIODACTYLA. 359 



The premolar and molar teeth are usually dissimilar, the 

 premolars being one-lobed and the molars two-lobed; the last 

 lower molar of both the milk and permanent dentitions is 

 almost always three-lobed. 



The grinding surfaces of the molar teeth have a tendency 

 to assume one of two forms. In the Pigs and their allies the 

 crowns are bunodont 1 , while in the more highly specialised 

 Ruminants the crowns are selenodont 1 . The nasals are not 

 expanded posteriorly, and there is no alisphenoid canal 2 . The 

 thoraco-lumbar vertebrae are always nineteen. The symphysis 

 of the ischia and pubes is very elongated, and the femur has 

 no third trochanter. The limbs never have more than four 

 digits, and are symmetrical about a line drawn between the 

 third and fourth digits ; the digits, on the other hand, are 

 never symmetrical in themselves. The astragalus has pulley- 

 like surfaces both proximally and distally, and articulates with 

 the navicular and cuboid by two nearly equal facets. The 

 calcaneum articulates with the lower end of the fibula when 

 that bone is fully developed. 



In the Artiodactyla are included the following living 

 groups : 



a. Suina. Pigs and Hippopotami. 



b. Tylopoda. Camels and Llamas. 



c. Tragulina. Chevrotains. 



d. Ruminantia or Pecora. Deer, giraffes, oxen, sheep 

 and antelopes. 



Suborder (2). PfiRissoDACTYLA 3 . 



In this group there are never any bony outgrowths from 

 the frontals. The grinding teeth form a continuous series, 

 the posterior premolars resembling the molars in complexity, 

 and the last lower molar generally has no third lobe. The 



1 See p. 345. 2 See p. 401. 



3 See E. D. Cope, "The Perissodactyla," Amer. Natural, 1887. 



