These four families are closely allied to each other, especially the Amphiumidae and the Salamandridae. The geographical distribution of the Urodela is essentially Periarctic, except that about one dozen species each of _Amblystoma_ and of _Spelerpes_ extend southwards into Central America, and in the case of the latter genus even into the Andesian parts of South America. _Plethodon platense_ inhabits Argentina. The Urodela afford good reasons for dividing the Periarctic region into three co-ordinate sub-regions, namely, Nearctic, Eastern and Western Palaearctic. The difference between the European and the Eastern Asiatic fauna is well marked; the two are–at least with our present knowledge–separated by a wide stretch of country very poor in Urodele forms; while, lastly {95}there are not a few resemblances between this Eastern Asiatic and the American fauna. The Urodela thus lend no support to the usual division of the Periarctic into a Palaearctic and a Nearctic sub-region. Nor is it possible to divide the Palaearctic into a Eurasian and a Mediterranean province. We have in this case to distinguish between an American, an Asiatic, and a European fauna. The Asiatic or Eastern Palaearctic sub-region assumes the central position, at least from a merely geographical point of view. It would be unjustifiable to assume a spreading from this centre into Europe, and, on the other hand, into America. The centre existed more probably in the Arctic circle, now devoid of Urodela. [Illustration: FIG. 16.–Map showing the distribution of the Urodela. "Ichthyodea" = Amphiumidae + Proteidae + Sirenidae.] So far as mere numbers of species are concerned the huge Asiatic or Eastern Palaearctic region is the poorest, but it is also the least explored, and China will probably yield a good many new forms. We know at present only 15 species, nearly all from the eastern half. These 15 species represent no less than 11 genera, 8 of which (= 73 per cent) are peculiar to the sub-region. Next comes the Western Palaearctic or European sub-region with about 21 recent species of 5 genera, 4 of which are peculiar. America is by far the richest, with no less than 66 species (36 eastern, about 16 western, and the rest Central American, etc.), belonging to 19 genera, 17 of which (= 90 per cent) are peculiar to the New World. But this richness in species is due mainly to the abundance of the two genera _Amblystoma_ and _Spelerpes_, just as Europe is characterised by its many Tritons. One of the most striking features of the Asiatic sub-region is {96}its difference from the European. They have very little in common. _Pachytriton_, _Tylototriton_, and two species of _Triton_ (_T. pyrrhogaster_ and _T. sinensis_) are the only Salamandrinae, while all the rest are Lechriodont (see p. 102), like the American Urodela, excepting the two American Tritons, _T. torosus_ and _T. viridescens_. GEOGRAPHICAL DISTRIBUTION OF THE URODELA