Article XX. A PRELIMINARY STUDY OF THE SOUTH 



AMERICAN OPOSSUMS OF THE GENUS 



DIDELPHIS. 



By J. A. ALLEN. 



INTRODUCTORY. 



About a year since, I published x 'A Preliminary Study of 

 the North American Opossums of the Genus Didelphis.' 

 Shortly after, I had opportunity to study the material relating 

 to this genus contained in the British Museum, through the 

 kindness of Mr. Oldfield Thomas, the Curator of Mammals. 

 The present paper is based almost wholly on the material 

 thus generously placed at my disposal. It includes speci- 

 mens from many different and widely separated localities, 

 some of which are represented by a considerable number 

 of specimens. Among them are practically topotypes of 

 Azara's ' Micour6 premier,' and of the forms described by 

 Lund and von Ihering from Southern Brazil. I also had the 

 pleasure of examining the types of Bennett's Didelphis cali- 

 fornica and D. breviceps. 



In a group so notoriously subject to variation due to age 

 and individual differentiation, it is necessary to examine a 

 very large amount of material in order to reach satisfactory 

 results. That now available is only sufficient for a preliminary 

 survey of the field, and the conclusions reached are neces- 

 sarily tentative, and are presented with some hesitation, 

 although the material studied by me at the British Museum, 

 combined with that in the American Museum of Natural 

 History, is probably greatly in excess of that examined by 

 any previous investigator of the group. Having in my 

 former paper considered in some detail the subject of sexual 

 and individual variation, and variations due to age, it is- 

 sufficient in the present connection to say that the South 

 American forms, as would be expected, vary in these respects 



1 This Bulletin, XIV, pp. 149-188, pll. xxii-xxv, June 15, 1901. 

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