ON THE IDENTITY OF THE IRISH RAT, 



3 



portance to this white breast-spot ; but it is needless to say 

 that, beyond being an interesting fact, this peculiarity possesses 

 no real value. 



This patch of white is borne upon the breast and between 

 the fore limbs. It varies much in size and shape, and is mostly 

 unsymmetrical, being more developed to the right or left of the 

 median line of the breast, as the case may be. In some it takes 

 the form of a narrow stripe extending backwards towards the 

 abdomen, and is an inch or more in length. In others it is a spot, 

 or a somewhat triangular and irregularly shaped patch. One 

 specimen examined in April, 1890, had three spots on the breast 

 and between the fore-legs arranged thus : — O O. But, as 

 above remarked, beyond being an interesting fact, this peculiarity 

 possesses no specific value ; on the other hand, similarly 

 marked examples of M. decumanus are unknown to us, though 

 we have come across some curiously marked specimens of that 

 species, in which the head and legs were flecked with white. As 

 Messrs. Thompson's and Eagle Clarke's descriptions have already 

 been reproduced in ' The Zoologist' (1889, pp. 201— 206), it is 

 not thought necessary to repeat or modify them here. 



The following tabulation gives the dimensions of several 

 individuals selected to show the variation to which mature 

 specimens of Mus hibernicus are subject: — 





3 



o* 



<? 



? 



$ 



2 



Length of head and body 



,, head 



In. 



8-4 



2-0 



7-4 



0-75 



0-8 



1*6 



Id. 

 9-5 

 23 



7-7 

 0-9 

 0-85 

 1-8 



In. 

 10-0 

 2-6 

 7-10 

 1-1 

 1-0 

 1-9 



In. 



8-25 



2-0 



7*7 



0-75 



0-75 



1-7 



In. 



9-0 



2-15 



6-1 



0-9 



0-8 



1-58 



In. 



10-20 



2*2 



,, tail 



8*5 



,, ears 



0-9 



,, fore-feet and claw ... 

 „ hind-feet and claw... 



0-8 

 1-75 



The heaviest male weighed 17j oz. The largest female, a 

 splendid specimen, had been gutted before it was received, and 

 hence no weight could be taken. 



Regarding the cranial characters, we find that there are two 

 well-marked types of skull represented in our British rats. The 

 most conspicuous difference between these types lies in the 

 interparietal, parietal, and squamosal bones, and the zygomatic 

 arches. To put it in a general way, in Mus rattus and Mus 



u 2 



