I po i.] Allen, Opossums of the Genus Didelpkis. 155 



the range in total length being from 122 to 139 mm., and in zygo- 

 matic breadth from 63 to 69 mm. ; the corresponding ranges for 

 the females being, respectively, 114 to 120 mm., and 55 to 

 62.5 mm. The ratio of the two measurements ranges from 48.3 to 



56.4 in the males, and from 46.4 to 55 in the females. 



From the preceding tables it will be seen that the largest skulls 

 are dispersed over a wide geographic area, and that most of the 

 extremely large skulls are from Mexico. This is due in part 

 doubtless to the fact that in the unsettled portions of that region 

 they are subjected to less persecution from man and thus more 

 frequently live to very old age, than in the more settled regions 

 of the United States. At all events, skulls bearing the marks of 

 extreme senility are more frequent in the Mexican material than 

 in that from the United States; yet certain skulls from North 

 Carolina, Mississippi, and southern Texas stand well toward the 

 head of the list, as regards both size and age. 



Taking the whole series of skulls into consideration, it is found 

 that some obviously very <?/^/male skulls measure down to 112 X 



58.5 mm. (Armeria, Colima), and even as low as 107.5 X 52 mm. 

 (San Juan Capistrano, Zacatecas), while some ' young adult ' 

 males fall as low as 95 x 46 mm. (Brownsville, Texas). Very old 

 female skulls sometimes measure as low as 103 x 47 mm. (Teapa, 

 Tabasco), and ' young adult ' female skulls as low as 90 x 44 mm. 

 (Mazatlan, Guadalajara, and Papantla. ) The extreme range in 

 very old males is therefore from 112 to 139 mm. in total length, 

 and from 58 to 69 mm. in zygomatic breadth; and in very old 

 females from 90 to 120 mm. in length and 40 to 62 mm. in zygo- 

 matic breadth. This indicates the amount of normal individual 

 variation in cranial measurements that may be expected in any 

 large series of skulls, even, it may be added, from practically the 

 same locality. 



Variation due to Age, etc. 



There is not only a wide range of sexual, individual, and age 

 variation in size and proportions in the skull considered as a 

 whole, but, as would be naturally expected, also in its various 

 parts, as in the form and relative stoutness of the zygomatic 

 arches, the length and form of the palate, the nasals, etc., and 

 in the size of the teeth. Without going into details, except in 



