468 MAMMALIA. 



coidal placenta. Tn the male the testes are temporarily placed 

 in the scrotum, at other times they lie in the ahdomen. 



The chief families are the Muridce (Eats and Mice), the 

 Arvicolidai (Voles), the Leporidce (Hares and Eabbits), the 

 Cavida (Guinea-pigs), the Castoridm (Beavers), and the Myo-x- 

 idm (D^ . mice). All we need consider are the Mice, Eats, Voles, 

 and Eabbits. 



MuRiD^E, OR Eats and ;Mice. 



In this family the tail is long as a rule, and the body is 

 long and narrowish. Hind-legs longer than fore-legs. Head 

 pointed. The back molars possess a tuberculate crown, entirely 

 covered with enamel. External ears clearly seen. The genus 

 Mus (Eats and Mice) have long, scaly, ringed tails. 



Three species of Ptats are found in our country — namely, the 

 Black Eat {Mus raftus), the Brown Eat (J/, decumavus), and the 

 Irish Eat {M. hibernicus) ; this last species, told by the diamond- 

 shape patch of white in front, is only found in Ireland and the 

 Outer Hebrides. The Black Eat only is indigenous to Europe 

 (unless M. hibermcnt: is really a distinct indigenous species) ; it 

 is smaller than the common brown rat, which can.e over to 

 Europe in the eighteenth century. It was imported into 

 England from India in shipping, and has now spread all over 

 the world. It may also have entered Europe from Asia by 

 migrating into Eussia. In any case, it has increased with 

 alarming rapidity, and in most places has quite driven out the 

 indigenous black rat. The black rat is smaller and much darker 

 than M. decumanus, which has a dusky-grey belly. 



The Eats live upon all manner of substances, and are noxious 

 in many ways,— a great nuisance indoors, a veritable pest in 

 poultry -yards and amongst corn, whilst they also distribute 

 the trichinosis amongst pigs, a disease which rats are very 

 subject to. 



A fourth species, Mu» alexandrinus, is sometimes found itt 



