1906.] Allen, Mammals from Western Mexico. 233 



gradually deepens, while at the same time the interorbital breadth 

 increases. In middle-aged male skulls the average interorbital breadth 

 is 3 . 5 mm. greater than the postorbital, while in old male skulls 

 it is 4 mm. less, resulting in a most marked change in the contour 

 of the middle region of the skull. 



The female apparently never develops even an incipient sagittal 

 crest ; the occipital crests are much less pronounced than in the male ; 

 the postorbital processes are much less prominent, and there is no 

 decrease in the postorbital breadth of the skull, so marked in the male. 

 The increase in the interorbital breadth is only in ratio with the gen- 

 eral increase in the size of the skull. The modifications due to sex 

 and age are shown in Plates XXVII-XXX, and in the subjoined 

 table of measurements. 



In preparing this table of measurements, only fully adult skulls 

 were utilized, and they were divided into four categories, as follows: 

 (i) old males (8 specimens) with well-developed sagittal and occipital 

 crests and more or less worn teeth; (2) fully adult males, middle-aged 

 (6 specimens), without 'crests and with unworn teeth; (3) old females 

 (8 specimens), all with the teeth greatly worn; (4) adult females, mid- 

 dle-aged (6 specimens) , with the teeth unworn. Younger specimens 

 (amounting to more than half of the total series), in which the milk 

 dentition was in most cases not wholly displaced, were discarded 

 as unavailable for comparative measurements. The principal points 

 thus shown may be summarized, as follows: 



Old males, 9 skulls: Total length, 135.3 (131-142.5); basal length 

 (Hensel), 118.9 ( I][ S- I2 5); palatal length, 80.8 -(77-86); postpalatal 

 length, 38.5 (37-40.5); zygomatic breadth, 73 (65-83); inter- 

 orbital breadth, 30 (26.5-33); postorbital breadth, 25.8 (23-28.5); 

 mastoid breadth, 49.5 (47.5-51.5) ; audital bulla (transverse and longi- 

 tudinal diameters), 11.5 x J 4- 2 ( IO -5 X 12-11 x X 5); premolar 4 - 

 molar 2 (last three molariform teeth), 22 (20.7-23); height of sagittal 

 crest (in 7 out of 9 specimens), 6.9 (5-8.5). 



Middle-aged males, 6 skulls: Total length, 130.1 (125.5-135.31); 

 basal length, 113 (108-116); palatal length, 78.3 (75-80); post 

 palatal length, 35.3 (33-37.5); zygomatic breadth, 63 (59-64); inter- 

 orbital breadth, 26.4 (25.5-27); postorbital breadth, 29.7 (28-31.5); 

 mastoid breadth, 46 (4448); audital bulla, 10.2 x I 3-2 (10 x I 3~ 

 ii x T 5); pm 4 -m 2 , 22.2 (21-23.3); sagittal crest, o. 



Old females, 8 skulls: Total length, 127 (121-133); basal length, 

 111.3 (108-120); palatal length, 74.6 (70-78); postpalatal length, 

 35-3 (34-37) ; zygomatic breadth, 65 (60-69) i interorbital breadth, 28.0 



