Section I.--Sciuromorpha, or Squirrel-Like Rodents.

Molar dentition 4--4/4--4 or 5--5/4--4. In the latter case the foremost upper molar is small; the fibula is distinct, and never united, except in some cases where it is attached to the extremity of the tibia; the zygomatic arch is formed chiefly by the malar, which is not supported beneath by a continuation of the zygomatic process of the maxillary; collar-bones perfect; upper lip cleft; the muffle small and naked; tail cylindrical and hairy (except in Castoridae). Five families.