I5 2 Bulletin American Museum of Natural History. [Vol. XIV, 



surface of the head is tinged more or less strongly with dusky. 

 There is usually a more or less well defined white area at the 

 inner base of the ears, this with the white cheeks constituting 

 the only white areas on the head. Often, however, the ear 

 patches are obscured by a dusky wash. 



A study of the skulls reveals an amazing amount of variation, 

 dependent partly upon age and sex, but in large measure due to 

 individual variation; there is, however, a considerable amount of 

 geographical differentiation, most strongly shown in the form 

 of the nasal bones. The variation in the form and size of the 

 skull as modified by age is so considerable that it has been 

 deemed worth while to illustrate this feature, as well as to give a 

 few illustrations of individual variation, and of certain abnormali- 

 ties of dentition. The halftone figures here given (Plates XXII- 

 XXIV) are, with two exceptions (later duly indicated), one half 

 natural size, and are from excellent photographs kindly taken 

 for me by Mr. Frank M. Chapman, Associate Curator of the 

 Department of Mammalogy and Ornithology. 



The extensive array of measurements given in Tables I-VI at 

 the end of this paper include about 15 measurements each of some 

 200 skulls of the genus Didelphis, and 4 external measurements 

 of about the same number of specimens, taken in the flesh by the 

 collector. These measurements, with the accompanying ratios, 

 so well illustrate variations due to age, sex, and locality, as well as 

 affording means for comparison of five species and as many ad- 

 ditional subspecies, that they seem entitled to permanent record. 



While the amount of material at present available is too limited, 

 as already said, for even a very satisfactory reconnoissance of the 

 group as regards its geographic forms, its careful study has yielded 

 some results of considerable interest relating especially to the 

 skull, dentition, and external characters. 



SKULL AND DENTITION. 



The skull in the Opossums of the genus Didelphis is remarkable 

 for the great development of the sagittal and occipital crests, 

 which become very highly developed in old age in both sexes. 

 The deposition of bony matter appears to continue throughout 

 the life of the animal, the skull increasing in its principal dimen- 

 sions as long as the animal lives. At the time of the appearance 



