ALLEN: AMERICAN COLLARED LEMMINGS. 527 



inflated anteriorly. The teeth in fully adult animals of the same 

 relative age do not show appreciable differences of value. Neverthe- 

 less there are several rather striking peculiarities which hold good on 

 the average, as differential characters between the Hudson Bay and 

 the Alaskan specimens. In the former, the rostrum and nasals are 

 longer, the nasals taper very little in their posterior half, but hold 

 their breadth well, and end abruptly, either straight across, or with 

 a slight bevel to the median line. In the Alaskan rubricatus, the 

 nasals usually taper gradually in the posterior half, and though some- 

 times ending squarely across, are more often drawn to a median point. 

 The anterior boundary of the parietals on the upper side of the skull 

 is usually much more deeply notched at the midline in richardsoni, 

 and the interparietal, though rather rectangular as in rubricatus, 

 generally differs in having the anterior border more bracket-shaped, 

 with the median point in the same transverse plane as the lateral 

 points, and a shallow notch on each side (Plate 1, fig. 7). 



Measurements. — The five largest specimens (all females) obta ned 

 by Mr. E. A. Preble at the type-locality in 1900, measured as follows 

 . (all are in the collection of the U. S. Biological Sur^•ey) : — 



Average 145+ 13+ 19+ 



The skull of 106,322 measures: greatest length 32 mm.; basal 

 length 29; palatal length 18; nasals, medially 9.2; zygomatic breadth 

 20; interorbital breadth 4.5; mastoid breadth 14.8; upper molar 

 row (alveoli) 8.3; lower molar row (alveoli) 7.5. 



'Molt. — The specimens at hand are insufficient to show the precise 

 dates of change. A few aged specimens from the northwest coast of 

 Hudson Bay illustrate well the irregularity of this process in old 

 animals. For, whereas in younger specimens the new hairs of the 

 summer coat grow so rapidly that they blend with the white hairs of 

 the winter pelage, and thus give rise to great variation in mottling 

 with the gradual loss of the white, in old animals the growth of the 

 new pelage is slower so that it appears as a dorsal patch of short 



