28 COMPARATIVE ANATOMY 



anteaters. The teeth of adults are either absent or lack enamel and 

 roots. Dentition is limited to a single set. 



Order 3. Rodentia. The rodents are gnawing animals, such as 

 rats, rabbits, squirrels, guinea pigs, beavers, porcupines, gophers. Canine 

 teeth are absent, and the incisor teeth in both jaws grow continuously 

 throughout life. The cecum is very large. 



Order 4. Carnivora. The carnivora include the fossil creodonts, 

 the cats, dogs, weasels, bears, racoons, and seals. Each foot has four or 

 five toes. The canine teeth are sharp and elongated. The clavicle is 

 reduced or absent. 



LEMUR CATTA 



Fig. 24. — Lemur, a primitive Primate. (Redrawn after Shipley and McBride.) 



Order 5. Artigdactyla. Artiodactyls are such hoofed forms as 

 cattle, deer, swine, sheep, goats, and camels, llamas, hippopotamuses, 

 and giraffes which have an even number of toes on each foot. The third 

 and fourth toes are larger, and the second and fifth reduced or absent. 

 The stomach is complex and the cecum reduced. 



Order 6. Perissodactyla. The perissodactyls are hoofed forms 

 usually with an uneven number of hoofs, such as horse, ass, zebra, tapir, 

 and the rhinoceros. The third toe is the largest and the only one func- 

 tional in the horse. The enamel of the back-teeth is complexly folded. 



Order 7. Subungulata. Hoofed forms usually with plantigrade 

 feet. Subungulates are the elephants and mastodons, and the hyrax or 

 cony. The proboscideans such as the elephants have five toes on which 

 they walk. Their testes do not descend into a scrotum. Sirenians 

 (Manatee and Dugong) are a suborder of this group. 



Order 8. Cetacea. The cetaceans include whales, porpoises and 

 dolphins. They are aquatic mammals with fish-like bodies. Hairs and 



