All Chelonians lay white EGGS, round or oval, according to their kind, but the shape of the eggs of one set sometimes varies within the greatest limits. The shell varies from a parchment-like, flexible, scarcely calcareous cover to a hard, well-polished case. As a rule the eggs, imbedded in the ground, are hatched after a few months, but in some of the northern kinds, e.g. _Emys orbicularis_, the hatching is deferred until the next spring, the embryo's development being arrested during the winter. How such eggs, buried a few inches only below the surface, withstand the often very severe North German and Russian winter is a mystery. Whilst the plastron is generally flat, it is more or less concave in the males of many species, notably in _Testudo_, _Cistudo_, and _Emys_. The general conclusions which can be drawn from the present GEOGRAPHICAL DISTRIBUTION of the Chelonia are as few and unsatisfactory as those applying to the Crocodilia, since all the main groups of Chelonians, and many more extinct families, occurred together in bygone ages in the same countries, for instance in Europe. {332}[Illustration: FIG. 70.–Geographical distribution of Cryptodirous tortoises.] [Illustration: FIG. 71.–Geographical distribution of Pleurodirous tortoises.] The marine forms are naturally cosmopolitan, but the _Testudinidae_ are likewise cosmopolitan, except in the Australian region. The _Chelydridae_, now restricted to North and Central America, occurred formerly also in Europe. The _Pleurodira_, in Mesozoic times plentiful in Europe, India, and North America, are now restricted to South America, Australia, and Africa; the _Pelomedusidae_ to Africa, Madagascar, and South America; the _Chelydidae_ to South America and Australia. In the latter country all the Chelonians belong to the Chelydidae. The _Trionychoidea_, occurring since the Cretaceous epoch in North America, in Early and Mid-Tertiary times in Europe, are now restricted to North America, Asia, and Africa. The country richest in Chelonians is America; North and Central America together possessing representatives of all the families except the Pleurodira, and these we know to have died out there. The _Dermatemydidae_, {333}_Cinosternidae_, and Chelydridae are now restricted to the Nearctic sub-region (including Central America). Poorest in genera and species, all of them Chelydidae, is the Australian region, where no fossils of other families have yet been discovered. Europe, with its few Testudinidae, does not come into consideration; Asia has at least Testudinidae and Trionychidae, and in addition the solitary _Platysternum_ in Indo-China, representative of a family whose affinities with the Chelydridae again proclaim the validity of the Periarctic region. [Illustration: FIG. 72.–Geographical distribution of Trionychidae and Chelydidae.] ORDER I. ATHECAE.