2 THE ZOOLOGIST. 



the sake convenience, we shall allude to it as Mus hibernicus in 

 this communication. 



We cannot understand why Mus hibernicus has been con- 

 founded or associated with Mus rattus. In no respect does 

 it possess any of the characters of that species, which it can 

 only be said to resemble in colour. It has also been thought 

 to be a hybrid between Mus decumanus and Mus rattus, while 

 Mr. T. Southwell, of Norwich (Trans. Norfolk and Norwich Nat. 

 Hist. Soc. ii. pp. 419—421 ; Zool. 1889, pp. 321—323), has sug- 

 gested that certain rats, which he was inclined to regard as 

 examples of Mus hibernicus, were hybrids between Mus decumanus 

 and Mus alexandrinus. We can only say, with respect to these 

 opinions, that not one of the fifty-six examples of Mus hibernicus 

 which have been examined by us was found to possess, in the 

 smallest degree, any of the peculiar characters of either Mus rattus 

 or Mus alexandrinus. All the specimens of Mus hibernicus which 

 have come under our notice have been characterised by the 

 possession of (1) a tail decidedly shorter than the head and body, 

 (2) comparatively small ears, and (3) cranial characters agreeing 

 entirely with those of Mus decumanus. 



Where, then, are the peculiarities which have caused this 

 form to be associated with Mus rattus and Mus alexandrinus. 

 Both these species have (1) the tail decidedly longer than the 

 head and body, (2) the ears comparatively large, considerably 

 larger than in M. decumanus, and (3) the cranial peculiarities 

 markedly different from those of M . decumanus. If M. hibernicus 

 is related to either of these forms, should it not be the possessor, 

 to a greater or lesser degree, of some of their important and 

 characteristic peculiarities ? Yet it has absolutely none of them. 



Thompson's original description of Mus hibernicus is an 

 accurate one ; but it is necessary to add that the examination of 

 a much larger series of specimens than came under his obser- 

 vation makes it certnin that the white patch on the breast is 

 possessed only by a comparatively small number of individuals; 

 of the fifty-six specimens we have received thirteen only possessed 

 this peculiarity. Thompson undoubtedly attached much im- 



the difference in the length of the ears between Mus hibernicus and Mus 

 rattus is not due to the way in which the type-specimen has been prepared, 

 lie nhould be inclined to admit it as a species. 



