1926] Hall: Sl^ull of the Bodent Otospermophilus grcmmunia heecheyi 



367 



The least postorbital breadth shows an even greater difference in 

 ratios in the different skulls than does the least interorbital breadth. 

 It will be noted from table 2 that the total increase is 8.9 mm. or 1.7 

 mm. less than the increase in the least interorbital breadth. This is an 

 increase of 127 per cent. Table 2 also shows that the ratio of this 

 measurement decreases from the beginning to the end of the series. 

 In skulls H, K, L and N, however, the least postorbital breadth is 

 greater than in Q. This proves to be a rather constant difference 

 betvv'een adult and immature specimens. A series of fifty males and 

 another series of forty-five females, both including animals that ranged 

 in age from those just shedding their milk premolars to very old indi- 

 viduals, were arranged in order of j^oungest to oldest and then divided 

 into four groups on a relative age basis. Table 1 gives the average, 

 maximum, and minimum measurementf^ of each group of these two 

 series. Group 1 is the youngest and group 4 is the oldest. The female 

 series is believed to average slightly younger than the male series. 

 Both show an actual decrease in the least postorbital breadth with age. 

 It is generally known that decreases with age occur in several parts of 

 the skull of different mammals. Taylor (1918, p. 439), for instance, 

 has pointed out an actual decrease with age in the interorbital con- 

 striction of Aplodontia. 



TABLE 1 



Actual Decrease of Least Postorbital Breadth with Increase in Age 



Group 1 



Group 2 



Group 3 



Group 4 



Number of specimens 



Average 



Maximum and minimum 



Number of specimens 



Average 



Maximum and minimum 



8 



15.4 



14.4-16.8 



7 



16.4 



16.2-16.8 



14 



15.5 



15.0-16.2 



14.5 



16 

 15.4 

 16.0 



cf 



13 



16.3 



14.7-18.0 



13 



15.7 



14.9-16.4 



16 



15.4 



14.0-17.0 



15.4 

 13.8-16.0 



Parietals. — In adults each parietal is in contact with a frontal, 

 squamosal, mastoid, the supraoccipital, and the interparietal. The 

 relations of the frontal contact have been pointed out. The squamosal 

 overlaps the parietal for a considerable distance at all points where the 

 two are in contact. This is reflected in the measurements given in 

 table 2 for the width of the parietal bones dorsally. In reality the 

 width of the parietals is only slightly less than the width of the brain 

 case taken at a point just below the zygomatic processes of the squa- 



