DISTRIBUTION OF MARSUPIALS AND MONOTREMES 323 



as already stated, peculiar to the Neotropical Region, 

 where they seem to take the place of the absent Insecti- 

 vores. The Opossums are best arranged in two genera 

 only, Didelphys with about twenty-five species, and the 

 very distinct Water Opossum (Chironectes) with a single 

 species only. The Tree Opossums (Didelphys) of which 

 Mr. Thomas recognizes five sub-genera, are distributed 

 all over the Neotropical Region and a single species, 

 Didelphys virginiana, is a well-known inhabitant of the 

 southern United States, and is therefore unquestionably 

 a Nearctic mammal. Whether, however, it is certainly 

 indigenous there, a remnant of the Marsupial-life of the 

 tertiaries, or whether it may not have extended its range 

 northwards from Central America in more recent times 

 it is hard to say. Certain it is that the Virginian Opossum 

 is very closely allied to its South American representa- 

 tive Didelphys azarm, and by some authors is considered 

 merely as a sub-species. Besides the Virginian Opossum 

 five others of the same genus are registered by Alston as 

 occurring within the boundaries of the Central American 

 Sub-region. The remaining species of Didelphys are 

 distributed all over South America down to Paraguay 

 and La Plata, and one species at least occurs in Chili to 

 the west of the Andes. 



The Water Opossum (Chironectes) has likewise a wide 

 distribution — from Guatemala to Southern Brazil, but it 

 is absolutely restricted to the Neotropical Region. 



