be 
ae 
THE OPOSSUMS, 207 
the two colours at their junction. Other characters apparently 
much the same as in the last species. Length of head and 
body of male about 12 inches; of tail about 15 inches. 
_ Distribution —South-eastern Mexico to Paraguay. 
The specimen represented in Plate XXXIV. is the type of 
Didelphys derbiana, so named from having been originally in 
the collection of a former Earl of Derby at Knowsley. Mr. 
Waterhouse, its describer, believed it to represent a distinct 
species on account of its brilliant coloration, the well-marked 
white stripe on the withers, and the great relative length of the 
hairy portion of the tail. The soles of the hind feet are also 
peculiarly black, while the fore feet are furnished with white 
hairs, and the naked portion of the tail is clouded with brown. 
A larger series of specimens have, however, shown that such 
variations are merely individual and have no specific value. 
This species is one of those in which the young, as soon as 
they leave the teats, are habitually carried on the back of the 
mother, with their prehensile tails tightly twisted round her tail. 
This feature, it may be added, is common to the whole of the 
small forms constituting the next sub-genus, one of which has 
- asynonym (D. dorsigera) referring to it. 
VII. ASHY OPOSSUM, DIDELPHYS CINEREA. 
Didelphys cinerea, Temminck, Monogr. Mamm., vol.i., p. 46 
(1827); Thomas, Cat. Marsup. Brit. Mus., p. 342 (1888). 
ATicoureus cinereus, Lesson, Nouv. Table Régne Anim., 
Mamm., p. 186 (1842). 
Didelphys noctivaga, Vschudi, Fauna Peruana, Mamm., p. 148 
(1844). 
Didelphys waterhousei, Tomes, Proc. Zool. Soc., 1860, p. 58. 
Characters. 
With this species we reach the fourth sub-genus 
(Micoureus) of the true Opossums, which includes a consider- 
able number of species, and is characterised as follows :— 
