204 THE ZOOLOGIST.- 






Mus rattus| Mus alex- Mus Mus 
(from Bell).) andrinus.*|decumanus.| hibernicus. 
an. lins|) am) in. || ain. linn ine 
Length of head and wis Ty 30.) ois (4d). 9.» Seen 6 
» of head 15 Ou) ee aeOs|| 2) Os eo mame 
>» of ears. Oreo eS | 30. 0 82 
Width of ears . ; Og soy. O27 oO mae 
Length of tail . Ape epi aC to eeu {jae He? gab 
;, of fore feet and claws Sl a0 Os 0 0 10 OS 
» of hind feet andclaws.| 1 43 a 36 ys 
The four species readily fall into two groups :— 
1. The Long-tailed and Large-eared species, in which the tail 
is longer than the head and body, and the ears comparatively 
large. M. rattus and M. alexandrinus belong to this section. 
2. The Short-tailed and Small-eared species, in which the tail 
is shorter than the head and body, and the ears comparatively 
small. M. decumanus and M. hibernicus belong to this section. 
This results in M. hibernicus being more nearly allied to 
M. decumanus than to M. rattus, with which it has hitherto been 
associated. This is most undoubtedly the case, and is borne out 
by a careful examination of the other characters of the two 
animals. Indeed, if specific rank be not conceded to M. hibernicus, 
then it must be regarded as a variety of M. decumanus. 
It is now necessary to state the differences which lead to the 
belief that M. hibernicus may be something more than a variety. 
Briefly stated, they are :— 
lst. It is a smaller and more elegant animal than M]. decu- 
manus, which is a much coarser creature in build and other 
characters.t 

* From H.M.S. ‘ Devastation.’ Forwarded in the flesh by Capt. J. R. N. 
Macfarlane, R.N. 
+ Since the above was written I have been much indebted to Mr. G. 
Barrett Hamilton, of Kilmanock, Co. Wexford, for many valuable notes. 
Mr. Hamilton informs me that he has had specimens of M. hibernicus equal 
in length to ordinary specimens of M. decumanus, but that the former were 
always lighter in weight; he also tells me that the head and tail are 
proportionately longer in M. hibernicus than in M. decumanus. I think it is 
possible, however, that melanic varieties of M. decumanus may sometimes 
be confounded with M. hibernicus. 
