GSG 



.Mit. J. r-Ewis noxiiorv: dx Tin: 



[Apr. Ill, 



but ill cliV-ssifviii^f tlicin, i-clijuicc was only pljicotl on tlu- (•Ii;u:ictor.s 

 noted above, whicb allouiMl no po.ssiltility ot" doubt. 



[Miis norrcgicKs, on the other liand, proved very constant and 

 no variation in colour was noticealile. In this species, tlie liaiis 

 of the uniler[>arts have .slate-f^rey bases and the u[)per surface of 

 the hind feet is pure white, a combination of characters a])par- 

 ently not found in Jf. rattus from E<ryi)t. 



Apart from this distinction, J/, norvcyicws is a larger and more 

 stoutly built animal ; the length of the tail never exceeds that of 

 the head and body, and the ears are shorter and more rounded. 

 There are also some easily recognisable skull dift'erences, which 

 need not be gone into here.] 



Everv specimen was carefully measured in the flesh by myself 

 with the same callipers, so that errors from a comiiarison of 

 measurements are reduced to a minimum. The measurements 

 taken were: snout to root of tail, tail, hind foot, ear. 



The lar<;est measurements in mm. were as follows: — 



[These measurements are given only as showing the greatest 

 sizes attained ; they are those of individuals and in no sense 

 average measurements, so that, for instance, it must not be 

 assumed that the eai'S of M. alexandriniia are on the average 

 larger than those of M. tectormn. Owing to the difficulty of 

 accurately estimating the age of any individual, it is impo.ssible to 

 give average measurements.] 



The proportionate lengtlis of the body and tail measurements 

 were found to vary in both species, but in M. iiorvi'ijlcnfi the tail 

 never e.xceeded the body length, and in J/, rattus the tail was 

 alwavs longer than the body length. 



The most relial)le measurement therefore on which to make 

 com]tari.sons and the only one which dealt witii a structural 

 feature and for that reason admitted of least error, was tliatof the 

 hind foot. 'Phis measiu'tiinent m.-iy therefore be taken as .-i fair 

 criterion of general size ami as forming a good and reliable index 

 for comparison. 



Curves of this measurement were m.-ide. and on refi'iring to 

 that of Mus rattiia ('I'able I., text tig. r)K). we note that the male 



