CHORD ATA — VERTEBRATA — MAMMALS 38 1 



Rocky Mountains. (2) Fissipedia (Upper Eocene to present) ; 

 last premolar above and first molar below, called sectorials, 

 always specially modified for cutting and bruising ; in front of 

 these the teeth are always compressed and pointed; behind 

 them they have broad, tuberculate surfaces. The dog tribe 

 (Canidae) made its appearance in the Eocene, passing through 

 the greater part of its development in North America. The cat 

 tribe (Fehdae) is first known from the Oligocene; of these the 

 huge saber- tooth tigers Machairodus and Smilodon (Fig. 164, 7), 

 living in Eurasia and in North and South America, survived 

 from the Miocene to the Pleistocene. The raccoons (Miocene 

 to present), a North American family, were probably derived 

 from the dogs in the Oligocene. The bear tribe (Ursidae) came 

 in with the Miocene ; it originated in Eurasia and did not reach 

 North America until the Pleistocene; during the Pleistocene 

 the great cave-bear (Ursus spelceus) of Europe was hunted for 

 food by contemporaneous man. The different branches of the 

 Fissipedia converge as they are traced back into the lower 

 Tertiary, pointing to a common ancestor in the Eocene. (3) 

 The Pinnipedia (Miocene to present) have limbs adapted to 

 aquatic life. They may have descended from forms allied to 

 the creodont Patriofelis. They are represented at present by 

 the seals and walruses. 



Order 6, Rodentia {Rodents) 



Small, fur-covered, vegetable-feeding mammals without 

 canine teeth and usually with only two long, chisel-like, contin- 

 uously growing incisors in each jaw. The typical incisor, seen 

 in most modern rodents, such as rats, mice, squirrels and beavers, 

 has the enamel confined to a band upon the anterior face ; since 

 the remainder of the tooth consists only of the softer dentine, a 

 most efficient, continually sharpened chisel is the result (whence 

 the name from Latin rodens, gnawing). 



The extinct Tillodontia (Lower to Middle Eocene of North 



