xxv. i-2 CLASSIFICATION 653 



widely open to the orbit, pentadactyle limbs, separate radius and ulna, 

 and so on. These features, being found in all early mammals, indicate 

 no closer affinity of the two orders than depends on evolution from a 

 common stock. It is not even clear exactly how the two groups are 

 related to the ancestral eutherians, and we must be content to say that 

 it is probable that animals with rodent specializations diverged from 

 the insectivoran eutherian stock in the late Cretaceous or Palaeocene 

 and then rapidly became differentiated into lagomorphan and rodent 

 types. The two orders are therefore placed together in an isolated 

 cohort Glires. 



2. Classification 



Cohort 2. Glires 

 Order 1. Rodentia (= Simplicidentata) 

 Suborder 1. Sciuromorpha 

 Thirteen families, including 



*Family Ischyromyidae. Palaeocene-Miocene. Eurasia, N. 

 America 

 *Paramys, Palaeocene-Eocene 

 Family Aplodontidae. Eocene-Recent 



Aplodontia, mountain beaver, N. America 

 Family Sciuridae. Squirrels. Miocene-Recent 



Scinriis, squirrel, Holarctic; Marmota, marmot, woodchuck, 

 Holarctic; Tatnias, chipmunk, N. America; Petaurista, 

 flying squirrel, Eurasia 

 Family Geomidae. Gophers. Oligocene-Recent. N. America 



Geomys, pocket gopher 

 Family Castoridae. Beavers. Oligocene-Recent. Holarctic 

 Castor, beaver 

 Suborder 2. Myomorpha 

 Nine families, including 

 Family Dipodidae. Jerboas. Pliocene-Recent. Palaearctic 



Dipus, jerboa, Eurasia 

 Family Cricetidae. Voles. Oligocene-Recent. World-wide 

 (except Australasia) 

 Peromyscus, deer mouse, N. America; Lemmus, lemming, 

 Holarctic; Microtiis, vole, Holarctic 

 Family Muridae. Rats and mice. Pliocene-Recent. Native to 

 Old World 

 Apodemus, field mouse; Rattus, rat; Mas, house mouse; Gits, 

 dormouse; Notomys, jerboa-rat 



