17 



and the muzzle somewhat pointed. The members of the genus are 

 natives of the Old World, throughout which, with the exception of 

 Madagascar, they are quite generally distributed. Nearly seven- 

 eighths of the whole number of species are commonly called rats. 



The distinction between rats and mice is arbitrary and based on 

 size. Exclusive of the tail, rats may be said to vary in length from 

 4J to 10 inches or more, while mice measure from 2 to 4 Indies. 

 With few exceptions, rats have six well-defined footpads (plantar 

 tubercles), the last on the hind foot being elongated in shape; the last 

 hind-foot pad of mice is usually circular (fig. 2). 



Of the many species of Mus only three or four have developed 

 the ability to adapt themselves to such a variety of conditions as to 

 become cosmopolitan. Four have found lodgment in America: The 

 common house mouse (Mus musculus}; the old English black rat 

 (Mus rattus}; the Egyptian, or roof, rat (Mus alexandrinus] ; and 

 the brown rat (Mus norvegicus), known also as the gray rat, barn 



FIG. 2a. Right hind foot of brown rat, showing long sixth foot pad. 

 FIG. 2b. Right hind foot of house mouse, showing round sixth foot pad. 



rat, wharf rat, sewer rat, and Norway rat. The black rat and the 

 roof rat differ from each other chiefly in color. Indeed some zoolo- 

 gists regard them as races of the same species, and the trinomial Mus 

 rattus alexandrinus for the roof rat is now in use among zoologists. 



DISTRIBUTION OF THE GENUS MUS IN AMERICA. 



The common house mouse ( M. musculus) found its way to America 

 soon after the first settlement by Europeans. It now inhabits all 

 settled parts of North and South America, as well as nearly the entire 

 Old World; but in very cold regions it does not always survive the 

 winters, and is therefore comparatively scarce or local. It almost 

 always reaches a new settlement sooner than the rat. 



The black rat (M. rattus} has been known in Europe since the 

 twelfth century. It was carried to South and Middle America about 

 three and a half centuries ago (1554). The time of its arrival in the 

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