ABSTRACT
The Rodentia are richer in species and individuals than any other order of mammals and so in a way may be termed the most successful mammalian order. The earliest known Rodentia dates from the Paleocene. One of the smaller rodent families, the Sciuridae are incompletely known in the Pleistocene. The tree squirrels are seldom found in the fossil state. The marmots have a fairly good record but only for the Late Pleistocene. The only group with an extended representation is the sousliks. The beaver is the largest living European rodent; it is about 8o cm. long, with an additional 32 cm. for the tail, and weighs up to 30 kg. The jerboas are bipedal rodents with greatly elongated shins and feet; the three middle toes are powerful, the side toes reduced or absent. This family has an exclusively Old World distribution; it appears in the Pliocene.
