ZOOLOGICAL POSITION OF THE ALBINO RAT. 



271 



FIG. 3. (X two diameters.)' The measurement of frontal-occipital length was 

 determined in the following way : 



Since the length measured from the tip of the nose to the posterior end of the inter- 

 parietal bone, is not always equal to the length measured from the tip of the nose to 

 the end of the occipital bone, both measurements were taken. First, the measure- 

 ment from the tip of the nose to the end of the occipital bone, and second, that from 

 the tip of the nose to the end of the inter-parietal bone. The difference thus obtained, 

 was added to the length of the frontal-interparietal line, and the sum was called 

 frontal -occipital length. 



The width of the cranium was determined by taking a maximum width between 

 jhe two points (right and left) where the zygomatic bones rest on the lateral walls of 

 the cranium. 



We conclude therefore, that the albino rats composing the 

 colonies at Chicago and Philadelphia, are similar to JMits nor- 

 vegicns in their bodily proportion, and in their cranial characters. 

 They are however, smaller in size than the specimens of J\fns 

 norvegicus usually found. 



