85 



NATURAL HISTORY. 



SECTION I.(SCIUROMORPHA.) SQUIRREL-LIKE RODENTS. 



FAMILY I. SCIUKIIXaS. 



This first family, which includes the true Squirrels and the Marmots, is distinguished from the 

 rest of the section by the possession of five rooted molars on each side of the upper jaw (see figure of 



COMMON SQVIUREL. 



the teeth on p. 85), the first being very small and sometimes deciduous, and four molars on each side of 

 the lower jaw, and by the presence on the skull and zygomatic arch of small processes, indicating 

 the posterior boundary of the orbits (see figure of the skull on p. 82). The molars are tubercular, 

 at least at first; but the summits of the tubercles are generally more or less worn down as the 

 animal increases in age. 



