228 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



Cranial characters. Skull, as compared with that of M. serpens, 

 is similar, but much less in the length of the zygomatic arch, 

 a greater constriction of the frontals anteriorly, and smaller audi- 

 tory bullae. 



Measurements. Adult; total length, 136 mm.; tail, 35; hind 

 leg, 15 ; ear, 7 ; average length of tail of five specimens, 34. 50 mm. 



Remarks. The young in some instances have bicolor tails, but 

 the under side apparently grows darker as they get older, until 

 in the adult both sides are practically of the same color. The 

 tail is apparently considerably longer than those of any of the 

 allied species, save M. oregoni Bach, whose tail exceeds that 

 of M. morosus by 8 mm., average. 



Zapus imperator. 



Type. Sieg's Ranch, Elwah River, Challam Co., Washing- 

 ton. Collected July 29, 1898. 



Characters. Similar to Z. princeps Allen, but even larger than 

 that species, and differently colored, being lighter both on the 

 back and sides. Hind legs and feet much stouter. 



Color. Top of head and upper parts to base of tail, brownish- 

 black mixed with buff, this latter color predominating between 

 the ears and back of head; sides of head, shoulders, sides of 

 body and outer part of thigh, bright yellowish-buff, sparingly 

 streaked with black hairs; entire under parts, and inner sides 

 of legs, with hands and feet, pure white. Tail cov.ered with 

 small scales; base, dark-brown above, white beneath. 



Cranial Characters. Skull is much broader for its relative 

 length than that of Z. princeps, with wider frontals and less ante- 

 orbital constriction. Nasals are longer, and broader at their 

 juncture with the frontals. Palate is shorter and narrower 

 posteriorly, and the pterygoids straighter. Auditory bullae, 

 somewhat larger. Molars are larger, the last upper molar being 

 about half the size of the one preceding. 



Measitrements. Type. Total length, 241 mm.; tail, 141; hind 

 leg, 28. In general appearance this species is much lighter 

 than Z. princeps, the only one with which it can be compared. 



