{346}This large family is cosmopolitan, with the exception of the Australian and the adjoining Austro-Malayan countries. It contains genera which form a continuous gradation between absolutely terrestrial and thoroughly aquatic tortoises; and many are truly amphibious. As a general rule the typically terrestrial kinds have a more curved or arched shell, the digits are short, the eggs are more oval or round, and they are chiefly herbivorous; the essentially aquatic kinds have a flatter or depressed shell, webbed feet, with longer, often slender claws, the eggs are more cylindrical, and they live on animal diet. About 20 genera, with more than 110 species, are recognised by Boulenger, but their essential characters are nearly all internal, and therefore of no avail for the determination of live or entire specimens. _Chrysemys._–One of the most typical and widely distributed genera of American Terrapins or water-tortoises. The carapace is flat; the plastron is quite immovable, with a strongly developed bridge. Feet well webbed. Tail short. Skull with a broad, complete, lateral, temporal arch. About one dozen species, mostly in the eastern half of the United States, but the whole genus ranges from Canada to Argentina. Most of the young _Chrysemys_ are very pretty, the ground-colour of the upper shields being green, variegated with yellowish-brown or blackish markings, which often form exquisitely delicate patterns, either concentrical (_Ch. concinna_, _Ch. rubriventris_), or more longitudinal (_Ch. elegans_), or apparently quite irregular. The ground-colour of the plastron is yellow, but the various species are best distinguished, at least in very young individuals, by the arrangement of the dark brown spots and patches. There are, for instance, several pairs of bold lateral and several median patches in _Ch. rubriventris_; five pairs of ocellated spots in _Ch. elegans_; only small median patches, where four plastral shields meet, in _Ch. concinna_; while the plastron of _Ch. picta_ is uniformly yellow.