1 90 1.] Allen, Opossums of the Genus Didelphis. 153 



of the first upper true molar(m 1 ) the skull (then about 45 mm. 

 long) is entirely smooth, without trace even of postorbital pro- 

 cesses; by the time the third upper molar (length of skull now 

 about 80 to 90 mm.) has come into use, the sagittal and occipital 

 crests appear as low ridges about a millimetre in height. By the 

 time the last upper molar has become functional the sagittal 

 crest has attained a height of 4 or 5 mm. and the occipital ridges 

 are proportionately developed. In middle-aged specimens the 

 sagittal crest has usually attained a height of about 8 to 10 mm., 

 which increases in old age to 12 to 14 mm., and the occipital 

 ridges form a broad posteriorly projecting crest. (See Plate 

 XXII.) 



Sexual Variation in the Skull. 



In the female the skull is of slighter build than in the male and 

 averages about 10 per cent, smaller in external dimensions. It 

 is narrower and slenderer, the zygomatic breadth decidedly less, 

 the muzzle narrower, and the canines less strongly developed; 

 nevertheless the sex cannot be distinguished with certainty from 

 an examination of the skull, owing to exceptional variations in 

 both males and females. Occasionally a female skull has the 

 large size and heavy development ordinarily seen in the male, 

 while, on the other hand, small male skulls are so slender and 

 delicate as to exactly resemble the skull of the average female. 

 In the general character of the dentition, aside from the usually 

 weaker canines of the female, there is little difference in the 

 sexes, and there is little or no difference in the relative develop- 

 ment of the sagittal and occipital crests. While it thus happens 

 that female skulls are sometimes larger than some male skulls 

 strictly comparable with them as to age, it may be said that nearly 

 all male skulls of the D. virginiana, D. marsupialis, and D. kar- 

 kinophaga groups exceed no mm. in basal length, and that nearly 

 all female skulls fall below this measurement. As indicating the 

 relative size of males and females, and the geographical distribu- 

 tion of the larger skulls in the present series, the following tables 

 may be of interest. 



The 21 largest male skulls out of a series of about 200 are here 

 listed, the two measurements given being the total length and 

 the zygomatic breadth, with the ratio of breadth to length. 



