454 Mr. M. A. C. Hinton on the 



closely resembles that of the adults of less modified members 

 of the arvalis group (e. g., M. a. levis) ; the squamosals are 

 separated anteriorly by a space equal to, or more than, the 

 least width of the interorbital region ; the squamous portions 

 of the parietals are small, overlapping the frontal s to a very 

 limited extent, so that the coronal suture forms a wide and 

 deep emargination of the parietals, while the posterior border 

 of the frontals is a broad semicircular process; the inter- 

 parietal is laterally unreduced, and has the short broad 

 form which characterizes it in M. agrestis and other compara- 

 tively primitive voles. 



In passing from youth to age the growth of the temporal 

 muscle moulds the outer surf ace of the brain-case, and induces 

 several important changes in the form of its component 

 bones. The temporal ridges gradually become more salient 

 and relatively higher in position. With the growth of the 

 anterior portion of the muscle the ridges eventually fuse into 

 a strong sharp crest in the interorbital region, and the latter 

 becomes relatively narrower ; the squamosals gradually 

 encroach upon and ascend the sides of the frontal at the 

 hinder end of the interorbital region, until at length their 

 upper edges are separated from each other by little more than 

 1 mm. ; the squamosal crest, whence the strong postorbital 

 tendon of the temporal arises, becomes more and more salient; 

 the squamous portions of the parietals are enlarged and, more 

 extensive^ overlapping the frontals than in the young, they 

 reduce the width of the notch formed by the coronal suture, 

 the posterior process of the frontals being correspondingly 

 reduced to a narrow projecting tongue (cf. the dimensions a, 

 b, and 3 in table at pp. 458 & 459). With the growth of 

 the posterior portion of the muscle the distance between the 

 ridges behind is diminished and the lateral processes of the 

 supraoccipital become more salient ; the supratympanic 

 fossas from which this portion of the m'uscle arises on either 

 side are therefore extended, and occupy a relatively wider 

 tract upon the dorso-lateral parts of the brain-case (cf. dimen- 

 sions 8, 9, and 9-8 in part ii. of the table at pp. 458 & 459). 

 The lateral extremities of the interparietal are consequently 

 reduced, and its growth is confined to longitudinal extension ; 

 by far its larger portion is situate between the temporal 

 ridges, and this portion acquires in consequence of the lateral 

 reduction a peculiar pentagonal outline in adults which is 

 very different from the form seen in young skulls of orca- 

 devsis or in adult skulls of less modified species. We have, 

 in conclusion, to note that in M. o. orcacLnsis the upper 

 border of the jugal is a boldly convex crest for the insertion 



