1899.] NATURAL SCIENCES OF PHILADELPHIA. 285 



General characters. — lu general ajjpearauce much like Microtus 

 arvalis from Braunschweig, Germany, but tail shorter, fur longer 

 and coarser, color paler, and skull longer and narrower. Enamel 

 pattern as in M. arvalis. Mammie, 8; plantar tubercles, 6; hip 

 glands conspicuous. 



Fur and Color. — The fur is harsh and coarse, that on middle of 

 back about 12 mm. in length. It is everywhere dark slaty plum- 

 beous at base. 



Ground color of dorsal surface buff, faintly tinged with Avood- 

 brown and coarsely ' lined ' with blackish brown. Sides clear 

 pale buff. Ventral surface creambuff much darkened, especially 

 anteriorly, bv the plumbeous bases of the hairs. Tail indistinctly 

 bicolor, soiled whitish below, brownish above. Feet dirty Avhite. 



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

 oped. Soles densely hairy behind tubercles. 



Mammce. — Mamm?e eight, four pectoral and four inguinal, as 

 usual in the subgenus Microtus. 



Skull. — The skull of Microtus ravidulus, though of the same 

 type as that of M. arvalis from Germany, is readily distinguishable 

 by its slightly greater length, and by the narrowness and great 

 depth of the braincase. The form of the braincase is much like 

 that of M. (^Pedomi/s) austerus. Interorbital constriction narrow, 

 the constricted region unusually long. Zygomatic arches not 

 widely flaring, the outer borders nearly parallel (not strongly 

 convergent anteriorly as in 31. arvalis) in the region opposite post- 

 orbital processes. Rostrum heavier than in M. arvalis, but not 

 conspicuously different in form. Audital bullae slightly larger than 

 in M. arvalis. Mandible similar to that of 31. arvalis, but larger 

 and with more slender angular process. 



Average measurements of three adult skulls: greatest length, 



26.3 mm. (26-26.6) ; basal length, 25.1 (25-25.4) ; basilar length, 



23.4 (23-23.6): zygomatic breadth, 14.4 (14-14.6); mastoid 

 breadth, 11.6 (11.4-12); interorbital constriction, 3; nasals, 7; 

 diastema, 8; palatal length, 14; occipital depth, 8; fronto-palatal 

 depth (at middle of molar series), 8.1 (8-8.4); mandible, 16.1 

 (16-16.4); maxillary tooth row (alveoli), 6; mandibular tooth 

 row (alveoli), 6. 



Teeth. — The teeth of 3Iicrotus ravidulus are relatively slightly 



