MICROTUS 663 



Diagnosis. — Size small or medium (hind foot, 17 to 21 mm.; 

 condylobasal length of skull in largest individuals, 24 • 6 to 28*4 

 mm.) ; second upper molar with well developed postero-internal 

 triangle ; first lower molar with four re-entrant angles on outer 

 side ; skull moderately broad (ratio of zygomatic breadth to 

 condylobasal length ranging from 53 to 60), the intertemporal 

 region developing a distinct ridge with age ; length of brain- case 

 measiu-ed from interorbital constriction to condyle equal to or 

 greater than zygomatic breadth. 



External characters. — General form robust, the neck short 

 and thick, the head large, wide and blunt, the ears inconspicuous ; 

 legs short ; tail about one-third as long as head and body. Head 

 large, broad posteriorly, tapering slightly to the blunt muzzle ; 

 ear scarcely appearing above fur, and extending, when laid 

 forward, barely half-way to eye, its outline evenly rounded ; 

 meatal lobe well developed, its height at middle about 3 mm. 

 Eye small, not very prominent, situated distinctly nearer to 

 muzzle than to base of ear, its diameter contained about ?>h times 

 in distance from inner canthus to muzzle. Nostril pad small, 

 not very well defined, divided down middle by a narrow but 

 evident groove continuous with that crossing upper lip ; nostril 

 opening almost directly outward, its anterior margin noticeably 

 swollen. Mouth small, the upper teeth very slightly projecting. 

 Fore foot with inner digit reduced to a minute tubercle, whose 

 dorsal surface is mostly covered by the closely appressed, highly 

 arched nail ; third and fourth digits sub-equal and longest, fifth 

 extending barely beyond base of fourth, second intermediate 

 between fifth and third ; palmar tubercles five, well developed, 

 occupying considerably more than half surface of palm, that at 

 base of thumb is slightly the largest (more than twice as large 

 as thumb itself), the others are sub-equal ; surface of palm between 

 tubercles slightly and inconspicuously granular. Hind foot 

 moderately long and slender, the toes relatively short (length of 

 third digit a little less than one-third length of sole) ; inner digit 

 extending scarcely to base of second, outer slightly beyond base 

 of fourth ; second, third and fourth sub-equal ; plantar tubercles 

 six, all well developed, occupying distinctly more than half of 

 surface of region in which they lie ; with the exception of the 

 sixth which is sub-circular in outline and only about half as large 

 as the others, they are sub-equal in size and all are irregularly 

 pyriform in outline ; surface of integument between tubercles 

 finely granular ; behind last tubercle the sole is densely covered 

 with short hair. Claws simple, moderately curved, those of hind 

 feet, toe for toe, slightly the longer. Tail extending slightly 

 beyond outstretched hind foot, its annulations pronounced though 

 often somewhat irregular, about 20 to the centimeter at middle ; 

 hairs not sufticiently abundant to conceal annulations, but form- 

 ing an evident though thin pencil, their length usually equal to 

 width of two or three rings. Fur without special peculiarities of 



