2 Rats and Mice 



2. Rattus norvegicus (" decumanus"). The Common 

 Eat or Bkown Eat (Plate 2). 



Size larger ; general build heavy and rather clumsy ; muzzle 

 blunt. 



^ars small, densely clad with fine and short hairs, thick and 

 opaque in substance, scarcely reaching the eyes when pressed 

 forwards. 



Tail stout, never as long as the combined length of the head 

 and body. 



Fads of soles of feet relatively small. 



Fur softer than in i?. rattus, the grooved bristles more slender 

 and in smaller number. 



Teats : the females normally with twelve mammae, three pairs 

 on the chest, three pairs towards the groin. 



Weight of adults normally from 14 to 17 ounces ; but specimens 

 weighing between 20 and 30 ounces have been frequently recorded ; 

 one mentioned in the Field (Sept. 20, 1913, p. 666) is said to have 

 weighed 2 lbs. 12 ozs. 



Measurements of two adults, in millimetres : — 



Head and Hind-foot, Ear, 



Body. Tail. without claws. from base. 



254 222 43 20 



267 229 45 20 



Good characters for the distinction of the two species are also 

 afforded by their skulls, which are described and figured on p. 59. 



It will be observed that nothing is said about colour in the 

 above comparison. The reason for this deliberate omission is the 

 fact that the specific determination of any given rat in Britain 

 must depend upon the characters mentioned above, and not upon 

 its colour. Inattention to this point has led many observers into 

 error ; because most " Black Eats " are brown, and many " Brown 

 Eats " are black. Colour only becomes important after we have 

 determined the species, when it is used to distinguish between the 

 sub-species or races. 



SUB-SPECIES AND HISTOEY OF THE BLACK EAT, 

 B. B ATT US. 



Three well-marked colour and pelage phases of this species 

 occur in Europe ; all three may be found living together in the 

 same colony, or may occur occasionally in the same litter in 



