IOSS I L RODENTS. 157 



tiated as at the present day, so far as the evidence goes ; and it is clear that we must go much 

 further back in time to seek the earliest appearance of the Rodent type, whether it branched off 

 directly from the Marsupial series, or passed, as would seem to be indicated by Mesotherium, 

 through a sequence of forms more or less related to the Ungulates. 



Nor does the geographical distribution of the animals lead to any more definite conclusions. 

 Certain families and even sub-families are of very wide range, the Muridae and Sciuridse especially 

 being represented nearly all over the world, while the Hystricidse and Leporidae are also spread over 

 very large areas, occurring in both hemispheres. Certain groups, such as the Sciuridse of both 

 sub-families, the Castoridse, the Murine, and Arvicoline sub-families of Muridae, the Leporidae, and the 

 Lagomyidse, may be said to have a circumpolar distribution in the northern hemisphere, nearly allied 

 and sometimes identical species being found in the more northern parts of both the Old and the New 

 World, but mixed with other forms peculiar to the regions, especially as we advance southwards. On 

 the other hand, the Myoxidae are peculiar to the eastern hemisphere, as are also the Spalacidae and the 

 Dipodidae (with the exception of Zapus, which is considered by Dr. Coues to form a distinct family), 

 and all the sub-families of Muridae, except those above mentioned as having a cinnimpolar range. The 

 Myomorpha may in fact be looked upon as an Old World group, the Geomyidae being the only 

 exclusively American family ; while the Hystriconiorpha as a whole may be regarded as American, 

 certain aberrant forms of the Octodontidae inhabiting various parts of Africa and the Old World 

 Porcupines being the sole representatives of that great section outside the western continent. Con- 

 sidering these facts, we may regard the Sciuromorpha and the Duplicidentata as originally polar 

 types, or at all events as having an equal claim to an origin in the northern regions of either continent ; 

 while the Myomorpha, with their multitudinous forms spreading over all parts of the Old World, and 

 having a much scantier representation in America, probably originated in the eastern hemisphere, and 

 spread by a northern passage into the New World ; and the Hystriconiorpha would seem to have 

 originated in South America, where they display the greatest variety of forms. 



W. S. DALLAS. 



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