454 MURIDA—ORCADENSIS 
adults of (in this respect) less specialised Grass Mice, such as 
M. hirtus; the inter-parietal is short and broad, the coronal 
suture widely though deeply emarginates the parietals, and the 
squamosals are widely separated anteriorly in the hinder part of 
the inter-orbital region (Fig. 72, 1 and 1a). As the muscles be- 
come stronger with age, the faint ridges, which low down on each 
side of the skull mark the origin of the temporal fascia, become 
more salient and gradually ascend so that the distance between 
them is steadily diminished. Finally, the growth of the anterior 
portions of the muscles causes the ridges to meet and fuse into 
a sharp inter-orbital crest in adults, just as in the agves¢zs group ; 
the anterior parts of the squamosals are stimulated to grow 
inwards and upwards upon 
the sides of the frontals 
until in the most advanced 
forms only the inter-orbital 
crest separates the right 
bone from the left; the 
post-orbital crests of the 
squamosals become more 
prominent, and each tends 

Fic. 72.—DoRSAL VIEWS OF SKULLS OF J/icrotus 
orcadensis. to develop an antero-ex- 
1. M. o. orcadensis, juy. (natural size); 1a, fore part ternal process Under the 
of brain-case x 2. 2. M. 0. sandayensis, adult; 2a, 
fore part of brain-case x 2; f frontals; sg. squamosal ; SAME stimulus the pari- 
p- parietal ; ir. temporal ridge. (Drawn by M. A. C. etals more extensively 
Hinton.) 
overlap the frontals ; con- 
sequently the coronal suture and posterior processes of the frontals 
are reduced to a narrow notch and narrow tongues respectively 
(Fig. 72, 2and 2a, and dimensions a, 6, and 3 of table at p. 462). 
The growth of the posterior portions of the muscles causes the 
supra- tympanic parts of the squamosals to encroach upon the 
region occupied in the young by the lateral parts of the inter- 
parietal ; the latter is therefore laterally reduced, and its growth 
becomes almost exclusively longitudinal ; its greater portion is 
situated between the temporal ridges, and it acquires in the 
adults of 0. orvcadensis and o. ronaldshaiensis a highly character- 
istic pentagonal outline; the lateral processes of the supra- 
occipital gradually increase in saliency and the supra-tympanic 
fossee, from which the hinder portions of the muscles arise, 
