520 bulletin: museim of comparative zoology. 



tips, and others wholly black. A clear tawny patch at the shoulder. 

 Sides and under siuf aces washed with ' ochraceous-buff ' ; chin 

 whitish. 



Sknll. — Compared with D. hud.soniu^, the skull shows a number of 

 differential characters; yet these are of an average nature and do not 

 hold in all cases where large series are compared. The tooth-char- 

 acters, however, are trenchant, and sharply separate the two species. 

 In ruhricahis the two anterior upper cheek-teeth (Plate 1, fig. 2) have 

 each an additional postero-internal fold of enamel and the last lower 

 molar an antero-internal fold (Plate 1, fig. 8) which are uniformly 

 lacking in hudsoniiis. In describing this species as nelsoni, Merriam 

 speaks of the small and narrow bullae as distincti\'e, but these \'ary 

 much in size in different specimens, and in a sufficient series of adult 

 skulls offer nothing characteristic. Adults attain practically the 

 same size in the two species, the characters of the jugal claimed by 

 Merriam disappear in a series, and there is no appreciable difference 

 in the length of the incisive foramina. But in general, ruhricatus has 

 shorter nasals, and slightly more squarely spreading zygomata; the 

 anterior boundary of the combined parietals is more deeply emarginate 

 medially, the interparietal is more nearly rectangular in outline, with 

 a short median prolongation, and the posterior arm of each parietal 

 usually extends as a narrow tongue between the squamosal and the 

 interparietal to the supraoccipital boundary (Plate 1, fig. 5). The 

 supraorbital ridges become prominent with age, and in some speci- 

 mens practically touch on the median line. In others they become 

 much thickened but preserve a shallow groove between. 



In a series of forty sktills of rubricatus, no less than thirteen show a 

 vacuity in the upper surface medially, between the frontals and the 

 parietals, and this is often retained even in adults. 



Measurements.^^ Five adults from Griffin Point, Alaska, were 

 measured in the flesh by Mr. Joseph Dixon, as follows : — 



Average 153 19 20 



The skull of a fully adult animal (M. C. Z. 11,850) measures: 

 greatest length 30.3 mm.; basal length 28.8; palatal length 17.3; 



