2/O SHINKISHI HATAI. 



To illustrate the differences found, both photographs and draw- 

 ings have been made. 



On comparing the skull of Mns rat/its with the brown rat, the 

 general un likeness can be seen in Fig. 2.' The most noticeable 

 difference is in the shape of the cranium. 



When viewed from the dorsal aspect, the cranium of Mus 

 rattns is oval in the outline, while that of Mus norvegicus is some- 

 what rectangular. Moreover, the dorsal aspect of the cranium 

 in Mns rattns is decidedly convex, while in Mus norvegicus it is 

 nearly flat. In Mns rattns the os nasale as compared to the 

 entire length of the skull, is relatively shorter than Mus norvegicus. 

 In J///Y rattns, the outline of the os interparietale is somewhat 

 semilunar in shape, while in Mus decumanus it is rectangular. 

 In J///.v rattns, the os parietale is broader as compared with its 

 length, than in Mus decumanus. In Mus rattns, the foramen 

 magnum is subcircular in outline, while in Hfus norvegicus it is 

 somewhat rectangular. On the ventral aspect of the skull, the 

 large tympanic bullae in Mus rattus are more conspicuous and 

 eminent than in Mus norvegicus. 



The junction point of the os basi-sphenoidale and os basi- 

 occipitale is flat in Ulns rattns, and protrudes in Mns norvegicus. 

 The anterior end of the maxilla which forms the lateral wall of 

 the infraorbital fissure, is blunter in M'us rattns, than in Mus 

 norvegicus. The skulls of our albino rats are very similar in the 

 above characters to those of Mus norvegicus, and the description 

 of J///.S- norvegicus may be taken to apply to them. 



In connection with the shape of the skulls, the determination 

 of a cranial index has been made. The index used, was that 

 obtained by dividing maximum width of the cranium by the 

 length of the fronto-occipital line. (See Fig. 3.) On account 

 of the small number of specimens measured, the accompanying 

 table is to be considered as merely preliminary, but as it stands 

 it shows a similarity in this index between Mus norvegicus and 

 the albino rats, and a difference between these two forms and 

 M'ns rattus. The cranial index will be made the object of a more 

 extended investigation. 



1 Care has been taken to use only the skulls of fully matured animals. See J. A. 

 Allen ('94) and II. C. Merriam ('95). 



