454 



MURID^— 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 \a). 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 agrestis 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 



Fig. 72-DoRSAL Views of Skulls of Muroius prominent, and each tends 

 orcademis. to develop an antero-ex- 



I. .!/. 0. orcadensis, juv. (natural size) ; la, fore part f ej-npl nrOCCSS Under the 



of brain-case x 2. 2. .1/. o. sandayensis, adult ; 2«, . 



fore part of brain-case y. 2 ; f. frontals ; sg. squamosal ; Same StimUlUS tttC pari- 



/. parietal ; tr. temporal ridge. (Drawn by M. A. C. etals more extensively 



Hinton.) i i /• i 



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, 2 and 2a, and dimensions a, b, 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. orcadensis and 0. ronaldshaiensis a highly character^ 

 istic pentagonal outline ; the lateral processes of the supra- 

 occipital gradually increase in saliency and the supra-tympanic 

 fossae, from which the hinder portions of the muscles arise, 



