MACGILLIVRAY'S GRASS MOUSE 



433 



in M. a. exsul ; 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 M. a. exsul) 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 exsul, 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). 



I i 3 « , 



Fig. 66.— Dorsal Views of Adult Skulls of Mkrotus agrestis Group. 



I. M. hirtus hirtus. 2. Af. a. macgillivraii. 3. M. a. exsul, 4. M, a. neglectus. 

 Natural size. (Drawn by M. A. C. Hinton.) 



The teeth agree with those of exsul ; 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, 10. 



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, ii-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, 

 i6-2 to 17.6; of mandibular tooth-row, 6-3 to 6-7. (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.) 



