322 THE ZOOLOGIST. 
size and length of tail approached /. decumanus: this I will call 
No. 1. On March 27th I received No. 2, which also resembled 
M. alexandrinus in having the upper jaw much longer than the 
under, but the total length of the head and body much exceeded 
that species, and the tail was shorter. The feet were strongly 
tuberculated ; eyes, large, bright and black ; general colour slate- 
black; hair long and coarse; a somewhat triangular or heart- 
shaped spot of white on the chest, and the fore feet white. No.1 
had not the white chest mark. A third specimen, received on 
May 9th, closely resembled No. 2, and had the white chest 
mark, but was even more robust. This last I sent to Mr. J. W. 
Clark, of Cambridge. Mr. Eagle Clarke’s figures in the ‘ Fauna 
of the Outer Hebrides, which I am glad to hear are to be repro- 
duced in ‘ The Zoologist,’ might have been drawn from Nos. 2 
and 3, above described. 
But for M. de l’Isle’s assurance that he could not induce 
M. rattus (which he considers specifically identical with M. 
alexandrinus) to breed with M. decumanus,* I should at once 
have regarded these Rats as hybrids between the Alexandrine 
Rat—an example of which had already been obtained from the 
same locality—and M. decumanus, and I am now strongly of 
opinion that such was the case; more than one generation had 
probably intervened in the seven months since the occurrence of 
the former species, and perhaps a further infusion of the native 
Rat would account for the greater similarity to the latter species. 
I am further inclined to this opinion from the examination of 
two Rats killed in the same neighbourhood in 1888, in which the 
apparent mixture of Alexandrine blood was slighter still. It 
may be that M. de I’Isle’s experiments at interbreeding under 
more favourable circumstances would have proved more suc- 
cessful. 
The sporadic occurrence of the recorded examples of the so- 
called Irish Rat, both as to time and locality, also tends, in my 
opinion, to show that the variety arose from the crossing of the 
Brown Rat with another species, whether it be M. rattus or M. 
alexandrinus appears to be a matter of indifference; probably 
both the Irish and the Orkney examples arose from a cross ~ 
* M. de l’'Isle’s paper will be found in the ‘Annales des Sciences 
Naturelles’ for 1865, pp. 173—222, 
