EVEN-TOED UNGULATES. 



693 



ARTIODACTYLA PIGS, CAMELS, 

 CHEVROTAINS, and RUMINANTS. 



PERISSODACTYLA TAPIRS, 

 RHINOCEROS, HORSES. 



The third and fourth digits of each foot 

 are equally developed, and the line 

 halving the foot runs between them. 



The premolars and molars are usually 

 different. 



There are nineteen dorso-lumbar verte- 

 brae. 



The femur has no third trochanter. 



The astragalus has always equal articular 

 facets for the navicular and for the 

 cuboid. The calcaneum has an arti- 

 cular facet for the fibula. 



The stomach tends to be complex, and the 

 caecum is small. 



The mammae are few and inguinal, or 

 numerous and abdominal. 



The placenta is diffuse or cotyledonary. 



The third digit occupies the middle of 

 the foot, is largest, and is symmetrical 

 on itself, so that the line halving the 

 foot bisects the third digit. 



The premolars resemble the molars. 



There are almost always twenty-three 

 dorso-lumbar vertebrae. 



The femur has a third trochanter. 



The astragalus has a large facet for the 

 navicular, a small facet for the cu- 

 boid. The calcaneum does not arti- 

 culate with the lower end of the 

 fibula. 



The stomach is always simple, and the 

 caecum is large. 



The mammae are always inguinal. 



The placenta is always diffuse. 



Sub-Order ARTIODACTYLA Even-toed Ungulates. 



Pigs and Hippopotamus (Suina), Camels (Tylopoda), 

 Chevrotains (Tragulina), and Ruminants (Pecora) like 

 Cattle and Deer. 



The general characters of this sub-order have been stated 

 above in contrast to those of Perissodactyla. The equal 

 development of the third and fourth digits, the fact that the 

 premolars have a single lobe while the molars have two, the 

 nature of the tarsal bones, the tendency that the stomach 

 has to be complex (as in Camels and Ruminants) are im- 

 portant characteristics. There are others of less obvious 

 importance, such as the absence of the alisphenoid canal, 

 which in Perissodactyla encloses the external carotid artery 

 as it passes along the alisphenoid. 



There are primitive extinct Artiodactyla which connect 

 the four modern groups Suina, Tylopoda, Tragulina, and 

 Pecora. Thus they unite the bunodont types, such as pigs, 

 with cone-like tubercles on the crowns of the molars, and 

 the selenodont types, such as cattle, with the tubercles 

 expanded from before backwards and curved in crescents. 



Group i. Suina Hippopotamus, Pigs, and Peccaries. The molars 

 are bunodont ; the third and fourth metacarpals and metatarsals are not 

 completely fused as "cannon bones." 



Hippopotamidne : Huge African mammals, included in the single 

 genus Hippopotamus. They spend the day in the rivers and 

 lakes, swimming and diving well, but usually remaining con- 



