268 



SH1NKISHI HATAI. 



These colonies have been recruited for the most part from the 

 northern states of the Atlantic seaboard, but some specimens 

 have come from as far south as Missouri. All the rats received 

 from these various localities have appeared to be of the same 

 variety, and have always bred true. 



Heretofore, the specific similarity of the albinos and the other 

 forms has been concluded from observation of the external char- 

 acters only. Wishing more exact information as to the zoologi- 

 cal relation of the rats composing these colonies, the present 

 investigation was undertaken to determine whether we were deal- 

 ing with an albino variety of Mus rattus or Mus decumanus. 



Externally, Mus rattus is usually distinguished from Mus uorve- 

 gicus by the following specific characters : 



Mus rattus is smaller in size. The tail of Mns rattus is con- 

 siderably longer than the body, while in Mus norvegicus it is 

 either shorter or only slightly longer than the body, but not rela- 

 tively as long as that of Mus rattus. 



The following measurements, though incomplete, serve to in- 

 dicate this relation : 



TABLE SHOWING LENGTH OF BOBY AND OF TAIL. 



The general shape of the head (see Fig. i) of 3 fits rattus is 

 slender, the nose is sharper, and the ear is both wider and longer 

 than in Mus norvegicus. It may be worth while to mention that 

 the so-called Alexandrian rat (Mus alc.vandrinus) is said to have 

 external characters similar to those of the black rat (Mus rattus) 

 and these two species are only distinguished by their coloring, 

 Mus alexandrinus having a brown colored coat. 



If we compare the external bodily characters of the albino rat 

 found in our rat colonies, with those of the brown rat, we are 

 surprised by their close similarity. All these characters of the 

 brown rat are also characters of the albino rats composing our 



