382 AN INTRODUCTION TO THE STUDY OF FOSSILS 



America and possibly of Europe) may belong to this order; 

 they have a primitive type of brain and retain traces of canine 

 teeth. True rabbits (Lepus) date from the Oligocene of North 

 America, the squirrel (Sciurus) from the Oligocene of North 

 America and Europe, the beaver (Castor) as well as the rats and 

 mice (Mus) from the Pliocene of Europe (Figs. 163, 5 and 164, 5). 



Order y, Edentata (Sloths, etc.) 



Dentition imperfect (i.e. incisors and canines usually absent, 

 premolars and molars without roots or enamel) or teeth entirely 

 absent (whence the name from Latin e, without, + dens, a tooth). 



These degenerate mammals may possibly have evolved from 

 the common ancestors of the rodents and ungulates through the 

 extinct Tseniodonta (Ganodonta) from the Basal to Middle 

 Eocene of North Africa. The earlier taeniodonts have well- 

 developed, rooted and more or less completely enameled teeth ; 

 in the later forms the teeth lose their roots and most of the 

 enamel. True edentates are known from the Eocene to the 

 present and from all continents except Australia. No fossil 

 remains of the modern sloths (Bradypodidae) and anteaters 

 (Myrmecophagidae) are known ; these two families are combined 

 in the extinct ground sloths (Megatheriidse) which have the head 

 and teeth of a sloth and the tail of an anteater (Fig. 164, 2) ; a 

 late American genus (Megatherium of the Pliocene and Pleis- 

 tocene) of this family is the largest-known edentate, one species 

 attaining a length of almost twenty feet. Armadillos are 

 found as early as the Eocene ; one of the largest genera known 

 is Glyptodon from the Pliocene of North and South America 

 with a rigid, usually ornate carapace ; this animal attained at 

 times a total length of fifteen feet. 



Order 8, Ungulata (Hoofed Mammals) 



Land-dwelling mammals with the weight of the body usually 

 resting upon the ends of the toes which are nearly always invested 



