194 LLOYD’S NATURAL HISTORY. 
moderately-sized blind appendage, or cecum. Pouch generally 
wanting, sometimes composed merely of two lateral folds of 
skin not united at the ends, and rarely complete. Three pairs 
of premolar teeth, the last of which always has a well-developed 
deciduous predecessor. 
The Opossums, which include more than a score of species, 
varying in size from that of a Cat to that of a Mouse, are con- 
fined at the present day to America, where they range from 
the United States to Argentina. ‘They may, however, be re- 
garded as more especially characteristic of the wooded districts 
of the southern half of that continent, but during the early part 
of the Tertiary period their range included England and other 
parts of Europe. Very similar to one another in external 
appearance, as well as in internal structure, the whole of the 
species, with the exception of a single one distinguished by its 
webbed feet and aquatic habits, are included in one large 
genus, which, however, may be split up into several subgeneric 
groups, thus rendering it less unwieldy than would other- 
wise be the case. As a whole, the family is regarded by 
Mr. Thomas as very closely allied to the Dasyuride ; one of 
its main claims to distinction being its isolated geographical 
distribution. 
The Opossums being among the lowest of Marsupials, the 
existence of a complete pouch among certain species is of the 
greatest importance as showing that this organ is primitively an 
essential characteristic of Marsupials, and not one that has 
been specially developed to suit the exigencies of the various 
modes of life of the Australian members of the order. Con- 
sequently those forms, like the majority of the Opossums and 
the Banded Ant-eaters, which have either no pouch or merely a 
rudiment thereof, may be safely assumed to have lost that 
organ through specialisation. ‘The reason of the loss of such 
an organ, which appears so admirably adapted for the protec; — 
