Allen, Opossums of the Genus Didelphis. 159 



Canines. No. 45401, adult male, Bay St. Louis, Miss., pre- 

 sents a case of the nearly equal development of two canines on 

 the left side (see PI. XXII, Figs. 5 and 53), not only similar in 

 size, but also in form, standing close together, one behind the 

 other. The two teeth have, at the alveolar border, nearly 

 twice the longitudinal diameter of a normal canine. 



CHANGE IN COLOR OF THE EARS WITH AGE. 



Perhaps the most interesting feature of color variation is the 

 change in the ears from flesh-color in the nursing young to black 

 in the adult. Very young animals, from whatever locality, have 

 the ears at first entirely flesh-color or yellowish white. In the 

 North this is gradually replaced by black until the whole ear be- 

 comes black, except the apical border, which varies from a mere 

 edging to an area half an inch in extent; while further southward, 

 from the Rio Grande region throughout Mexico and to northern 

 South America, the ears in the adult are entirely black. The 

 change from the flesh-colored ear of the young to the black of 

 the adult is gradual, the black generally beginning at the base 

 and extending upward, as illustrated by a number of young speci- 

 mens that have passed beyond the first stage. 



In this connection I may be allowed to correct an error based 

 on an examination of young specimens from Costa Rica which, 

 having particolored ears, I identified some years ago ' as Didel- 

 phis aurita. In the light of the present material, these specimens 

 prove to be merely in the intermediate stage of color change in 

 the ears, and have no relation to the true D. aurita of southern 

 Brazil, which in the adult has 'the apical portion of the ears flesh- 

 color, as in the Virginia Opossum of the North. 



NOMENCLATURE. 



The genus Didelphis of Linnaeus is retained for the present 

 group (cf. this Bulletin, XIII, 1900, pp. 185-190), the type being, 

 by elimination, D. marsupialis Linn. The specific name mar- 

 supialis is restricted to the Mexican species, for reasons given 

 beyond (pp. 163, 164); D. karkinophaga is retained for the forms 

 occurring in northeastern South America, and D. virginiana for 

 those of the United States. 



1 This Bulletin, III, No. 2, 1891, p. 217. 



