ROCKY MOUNTAIN PIKA 133 CHAPTER V. FOSSIL RODENTIA. Families of Rodents represented by Fossil Remains--State of the “Record of the Rocks”--THE SCIURIDÆ--Sciurine Genera now Extinct--No Fossil ANOMALURIDÆ and HAPLODONTIDÆ--ISCHYROMYIDÆ--_Pseudotomus hians_--_Gymnoptychus_--CASTORIDÆ--Mr. Allen’s CASTOROIDIDÆ--THE MYOXIDÆ--No Fossil LOPHIOMYIDÆ--THE MURIDÆ--THE SPALACIDÆ--THE GEOMYIDÆ--THE DIPODIDÆ--THE THERIDOMYIDÆ--THE OCTODONTIDÆ--THE HYSTRICIDÆ--THE CHINCHILLIDÆ--THE DASYPROCTIDÆ--THE CAVIIDÆ--THE LEPORIDÆ--THE LAGOMYIDÆ--_Mesotherium cristatum_--Difficulties concerning it--Mr. Alston’s Suggestion--THE HEBETIDENTATA--Teeth--Skull--Skeleton--Conclusions regarding it--Table of Rodent Families--Concluding Remarks 151 ORDER EDENTATA, OR BRUTA (ANIMALS WITHOUT FRONT TEETH). CHAPTER I. SLOTHS. The South American Forests--Discovery of the Sloth--How it derived its Name--Peculiarities of Dentition--Food--Fore Limbs and Fingers--Hind Limbs and Heel--Other Modifications of Structure--Kinds of Sloth--Waterton’s Captive Sloth--Habits of the Animal--Burchell’s Tame Sloths--Manner of Climbing Trees--Disposition--Activity among Trees--Naturalists’ Debate about Anatomy--Probable Conclusion regarding it--Skeleton--Vertebræ--the Rudimentary Tail--Most Distinctive Skeletal Characters--Arm, Wrist, Hand, Fingers, Claws--Mode of Walking--Great Utility of the Claws--Face of Sloth--Skull--Teeth--Classification--_TARDIGRADA_--BRADIPODIDÆ--Genus BRADYPUS--Characteristics--Genus ARCTOPITHECUS--Characteristics--CHOLŒPODIDÆ--THE COLLARED SLOTH--Description--Skull Bones--Habits--Circulation of the Blood--_Rete Mirabile_--THE AI--THE UNAU--Appearance--Skull and Teeth--Skeleton--Interesting Anatomical Features--Stomach--HOFFMANN’S SLOTH--Description--Habits 158 CHAPTER II. THE ANT-EATERS.