NORTH AMERICA AND THE WEST INDIES 41 



see no proof of more than one of Miocnus or Microcnus." So 

 far nothing seems to have been discovered that would indicate 

 that ground sloths in Cuba were contemporaneous with early 

 man. 



Order RODENTIA: Gnawing Mammals 



The rodents, here considered to include the rabbits and 

 pikas, are the largest order of mammals, both in number of 

 species and of individuals. They are characterized by chisel- 

 like incisor teeth, usually a single pair above and below, but 

 with a second pair of upper incisors in the rabbits. They are 

 mostly small mammals, with large populations and rapid rates 

 of reproduction. There are four suborders: 



(1) Lagomorpha (Duplicidentata) : The rabbits and hares, 

 found in all zoogeographical regions, except the Australian, 

 and pikas of Asia and western North America. Rabbits have 

 been introduced into Australia and elsewhere. 



(2) Sciuromorpha : This includes the squirrel family, which 

 is world wide, except for the oceanic islands and the Australian 

 Region; the scaly -tailed African "flying squirrels," the African 

 springhaas; the beaver, found in Europe, northern Asia, and 

 North America; and the sewellel of the Pacific coast of North 

 America. 



(3) Myomorpha: The rats, mice, jerboas, and their relatives; 

 representatives of this group occur in all regions, although 

 human agency is doubtless responsible for their presence on 

 some oceanic islands. 



(4) Hy stricomorpha : The porcupines, cavies, chinchillas, 

 and their relatives; representatives of this group are found in 

 all regions, except the Australian and the oceanic islands. 



The vanishing or extinct North American rodents are the 

 following : 



(1) Sciuridae, squirrels, four forms of a single genus. 



(2) Castoridae, beavers, eighteen races of the single Ameri- 

 can species. 



(3) Cricetidae, native rats and mice, eight species of three 

 genera. 



(4) Echimyidae, hystricoid spiny rats, twenty species and 

 subspecies of nine genera. 



(5) Heptaxodontidae, the unique species, thought to have 

 become extinct about the time of Columbus. 



