III.] RODENTS. 87 



lels (Haplodontida), with one genus ; and the Sonoran region the 

 pouched rats (Geomyidce) with two. It is further highly significant 

 that all of these families which are mainly or chiefly Neotropical 

 belong to one division of the order, the Hystricomorpha, and 

 that certain of them are represented either in the Santa Cruz beds 

 of Patagonia, or the Parana beds, where the whole of the other 

 sections and families are totally unknown. This, however, is by 

 no means all, for throughout the Tertiaries of North America 

 below the Pliocene there are no Hystricomorpha at all, and at the 

 present day there is but a single species the Canadian porcupine 

 (Erethizott) inhabiting the whole of that country. On the other 

 hand, both at the present day and in the Tertiaries, North America 

 abounds in Sciuromorpha and Myomorpha. These facts clearly 

 point to the existence of an impassable barrier between North 

 and South America during a large portion of the Tertiary 

 period. Moreover, even at the present day the Sciuromorpha 

 are scarcely represented at all in Neogaea, while the Myomorpha 

 are not very numerous. 



Of the squirrel-like rodents, constituting the section Sciuro- 

 morpha, and including the four existing families of the African 

 flying-squirrels (Anomalurid(z\ the squirrels and marmots (Sciur- 

 idcK], the sewellels (Haplodontida), and the beavers (Castoridce), 

 as well as the extinct American Castoroidida, the only living 

 Neogaeic representatives are certain species of squirrels, none x>f 

 which range south of Paraguay. The extinct Castoroididcz include 

 large beaver-like rodents with complex molars like those of the 

 viscacha, and are represented by the typical Castoroides from the 

 Plistocene of the United States, and likewise by Amblyrhiza 

 (Loxomylus) from caves in the West Indies, and, it is said, also 

 from the later Tertiaries of Argentina. This family is accordingly 

 a northern type. 



The second or murine group of rodents (Myomorpha) contains 

 five families, namely the dormice (Myoxidce), the jumping-mice 

 and jerboas (Dipodidce), the mice and rats (Muridce), the mole-rats 

 (Spalarida), and the American pouched rats (Geomytda). Out of 

 all these, practically the only one represented in the realm is the 

 cosmopolitan Muridce, although among the Geomyidce the genus 

 Heteromys enters the transitional Mexican sub-region. In the 



