EXPLANATION OF PLATES. 



Plate 1. 



Top row, a set of arbitrary grades used in the classification of the rats studied. 



Middle and bottom rows, skins of rats graded as indicated by the numerals above each 



skin. The animals graded, +4, +4} and +4j, being entirely dark above, are shown 



in ventral view. 



Plate 2. 



cfSOOO, a wild gray rat caught in Cambridge, Mass., October, 1908. 



9 6176, a black hooded rat of grade — Ij which was mated with d'8000. 



(^8021 and cfSOlS, Fi offspring of the pair just described, cTSOOO and 9 6176. cf8021 is 



of grade +41, with considerable white below. Notice also his white legs and compare 



with those of his father, d'8018 is of grade +5j. Notice white areas on belly and 



front legs. 

 8075-8078, four Fz progeny (grandchildren) of cfSOOO and 96176. 8076 is black hooded, 



grade -f ; 8076 is gray, +4i; 8077 is gray, +4; 8078 is black, +5i. 



Plate 3. 



cf 103, an evenly marked rat of grade +3. A female rat of this same grade was mated 



with d'8000 (the wild gray male). Two htters of Fi grandchildren are shown in 



8062-8067, and 8070-8074. 

 8062 is black hooded, +2; 8064 is gray hooded, +2; 8065 is black, +5; 8066 is gray, 



+5i; 8067 is gray, +4| (notice white feet). 

 8070 is black hooded, +2; 8071 is gray hooded, +lf ; 8072 is gray, +5; 8073 is black, 



+51; 8074 is gray, +5|. 



56 



