4 THE ZOOLOGIST. 



alexandrinus the cranium is broad and arched; while in Mus 

 decumanus and Mus hibemicus the cranium is straight and 

 depressed. 



These differences are well and accurately shown in the accom- 

 panying figures (p. 5), for which we are much indebted to the skil- 

 ful pencil of our friend Mr. Herbert Goodchild, of Edinburgh. 



The characters of these two types of cranium are expressed 

 and compared in the following tabulated form : — 



MUS EATTUS 

 AND MUS ALEXANDRINUS. 



MUS DECUMANUS 

 AND MUS HIBEENICUS. 



t~+™. „-^+«i„ f For difference of shape, see ( For difference in shape, see 

 Interparietals . \ „. * \ Vi * 



\ F 



gure 



Parietals 



Squamosals 



Zygomata 



Nasal region... 



( Convex ; broad between the 

 ( lateral ridges 



(The anterior temporal mar- 

 gins of these bones, and 

 the anterior margins of 

 the parietals, are in the 

 same line 



Tapers somewhat conspicu- 

 ously, as compared with 

 Mus decumanus and Mus 

 hibemicus. 



Figure. 



r Flat; narrow between the 

 lateral ridges, which are 

 closer to each other than 

 in Mus rattus and Mus 

 alexandrinus. 



The anterior temporal mar- 

 gins are considerably in 

 advance of the anterior 

 margins of the parietals. 



More expanded than in Mus 

 rattus and Mus alex- 

 andrinus. 



It is perhaps not improbable that Mus decumanus and Mus 

 hibemicus interbreed, but we have no evidence that such is the 

 case ; nor can the fact that parti-coloured specimens appear, so far 

 as we have been able to ascertain, to be most uncommon, be 

 urged fn proof that such unions do not take place. That evidence 

 of such a nature is perhaps not to be expected, we may refer to 

 the experiments of J. von Fischer (Zool. Garten, 1869, p. 341), 

 who attempted to breed a variegated form of M. musculus by 

 pairing grey with white specimens, but without success, the young 

 always being either uniform grey or uniform white. M. De LTsle's 

 elaborate experiments on the inter-breeding of Mus rattus and 

 Mus alexandrinus (Ann. Sci. Nat. iv., Zoologie) also prove that 

 parti-coloured specimens are of extreme rarity. 



