MACGILLIVRAY’S GRASS MOUSE 433 
in MW. a. exvsul,; the under-side is almost devoid of the yellowish wash 
often so conspicuous in that form, and, owing to the thin pelage, the slaty 
hair-bases are frequently visible and take part in the general coloration. 
The young have a dusky pelage in which there is very slight 
development of the light dorsal hair-tips. Those of the under-side are 
of a very light tint, increasing in depth towards the flanks and poll, 
where they are deepest, but never approaching the rich tints of adults ; 
the dusky rump is in contrast to the back. 
The skull (as compared with that of J. a. exsu/) is distinguished by 
its deeper rostrum, lighter jugals, and more nearly vertical occiput ; 
in correlation with the latter the interparietal is longer antero- 
posteriorly and has the posterior border straight instead of sinuous. 
Still more characteristic is the course of the temporal ridges on the 
anterior part of the brain-case, since they are never pushed backwards 
to the extent accruing in ersw/,in aged specimens of which they are 
constricted anteriorly; the shape of the “shield” delimited by these 
ridges is therefore very different in the two forms (Fig. 66, 2 and 3). 

Fic. 66.—DorsAL VIEWS OF ADULT SKULLS OF Microtus agrestis GROUP. 
1. MZ, Airtus hirtus. 2. M. a. macgillivraii, 3. WM. a. exsul. 4. M. a. neglectus. 
Natural size. (Drawn by M. A. C. Hinton.) 
The teeth agree with those of exsu/; m(in eight skulls examined) 
has constantly a fourth inner angle behind, and this feature is sometimes 
largely developed. 
Dimensions in millimetres :—Adult males (type, and two other old 
specimens): head and body, 125; tail, 32, 37, 40; hind foot, 19; ear, 
—, 15,12. Adolescent: head and body, 115; tail, 34; hind foot, 19; 
ear, 12. Young: head and body, 90; tail, 27; hind foot, 18; ear, to. 
Skull :—Condylo-basal length, 26-4 to 28-7; breadth, zygomatic, 
15-3 to 16-4; least inter-orbital, 3-6 to 3-8; mastoid, 11-8 to 12-6; 
occipital depth (median), 6-2 to 6-7; length of nasals, 7-6 to 8-7; of 
diastema, 7-2 to 7-8; of maxillary tooth-row, 6-6 to 7-1; of mandible, 
16-2 to 17-6; of mandibular tooth-row, 6-3 to6-7. (Except the least inter- 
orbital breadth, which decreases with age, the minimum values are those 
of immature, the maximum those of adult skulls, the subadults ranging 
between these extremes.) 
