NATURAL SCIENCES OF PHILADELnilA. 15 



■weeds, or little slivubliy patch, in a diy situation. It is sunk a 

 little in the ground, then arched over, with an opening on one side ; 

 built of dried grasses, mixed with a few leaves, and warmly lined 

 with the animal's own fur. All those I have seen were so far 

 from water as to be out of reach of an}'' possible rise of the tide. 



This appears to be the most distinct of the several species 

 closel}^ related to the foregoing ; and is in fact distinguishable on 

 sight, as some of the others are not. 



Didelphys virginiana, Shaw. 

 Syx. D. californica, Bennett, P. Z. S., i, 1833, p. 40. — B. brcviceps, Id., 

 ibid. — D. pruinosa, Wagner, Suppl. Schreber, ill, 1843, p. 40. 



Abundant in the wooded portions of the vicinity. 



Having made this animal a special study, I am in position to 

 offer remarks which cannot be reasonably questioned. Although 

 my investigations were rather anatomical than zoological, I did 

 not neglect the latter portion of the subject. The more speci- 

 mens I examined, the more I was struck with the variations that 

 depend upon sex and age, as well as those that different indi- 

 viduals corresponding in these conditions present. An examina- 

 tion of these points in the natural history of a single animal, may 

 give results of general application. And j^et, in calling attention 

 to the variability of the opossum, I do not wish to be understood as 

 supposing that the animal is not as constant as many or most 

 others ; for I believe it to be no exception to a general standard 

 or average in this respect. I doubt that one could study any 

 mammal as closel}' as I have the opossum, without being simi- 

 larl}'^ impressed. The following paragraphs are mainly confined 

 to consideration of external characters. I can affirm that not 

 one of the characters assigned to the supposed species above 

 cited, is not to be found in specimens of D. virginiana from the 

 same locality ; that the differences indicate individual peculiarities ; 

 and that even upon striking an average of preponderance of certain 

 characters, common to each in various degree, no results will be 

 obtained warranting the separation of the opossums from the 

 southwest as even a geographical race or variety. The assertion 

 is made, it should be understood, upon consideration of descrip- 

 tions onl}', without direct comparison of specimens. 



Professor Baird remarks (Mex. B. Survc}^, vol. ii, pt. ii, p. .33), 

 that "although there are some differences in the skulls of the D. 



1871.] 



