1899.] NATURAL SCIENCES OF PHILADELPHIA. 295 



Fur and color. — Fur full and soft, that on middle of back 10 

 mm. to 13 mm. in length. 



Dorsal surface pale smokegray, strongly tinged with woodbrown 

 and faintly darkened by a sprinkling of blackish hairs. Ventral 

 surface of body and back of feet pure white, irregularly clouded 

 by the plumbeous bases of the hairs. A narrow lateral Une ex- 

 tending down hind legs to ankles, crossing buttocks, and including 

 whole of tail, pale creambufF. Front legs white. 



Ears. — The ears are well developed, slightly overtopping the 

 fur. They show no pecuUarities in form. 



Feet. — Pahns with five tubercles, soles with six; all well devel- 

 oped. Soles densely furred behind tubercles. 



Skull. — The skull of Microtus cricetulus is in all respects that of 

 a typical Alticola. The brain case is rounded in outhne when 

 viewed from above, the interorbital region is broad, with a distinct 

 median furrow, and the nasals are very large. Postorbital pro- 

 cesses small but distinct. Viewed from the side, the skull is strongly 

 cuneate in outline, owing to the nearly flat dorsal profile, deep 

 brain case, prominent audital bullae, and slender rostrum. It differs 

 from the skull of M. albicauda, its nearest ally, in slightly larger 

 size, a little more highly arched brain case, and in the larger size 

 and more anterior position of the rostral protuberances. The nasal 

 branches of the premaxillaries are narrower than in M. albicauda. 

 They terminate on a line with the posterior extremity of the 

 nasals, while in 3f. albicauda they extend considerably behind the 

 nasals. 



Measurements of type skull: greatest length, 28; basal length, 

 26.6; basilar length, 25; zygomatic breadth, 16; mastoid breadth, 

 13.4; interorbital constriction, 4; length of nasals, 9; anterior 

 breadth across nasals, 4; posterior breadth across nasals, 2; dias- 

 tema, 9; palatal length, 13.4; occipital depth, 8; fron to-palatal 

 depth (at middle of molar series), 7; mandible, 17; maxillary 

 tooth row (alveoli), 6; mandibular tooth row (alveoli), 6. 



Teeth. — Both upper and lower incisors are very pale yellow on 

 their anterior faces, contrasting strongly with the deep orange 

 incisors of M. albicauda. Molars noticeably heavier than in 3f. 

 albicauda, but enamel pattern, with a single exceptioD, exactly the 

 same. In M. albicauda the inner base of the posterior loop of the 

 back upper molar is produced into a conspicuous salient angle. In 



