Throughout the greater part of Asia no Crayfishes are found until we come to the Far East, where we find an isolated colony in the river-system of the Amur, in Korea, and in the north of Japan. These far eastern Crayfishes, however, differ so much from the typical species of _Astacus_ that they are now placed in a subgenus (sometimes regarded as a distinct genus), _Cambaroides_. Curiously enough, the typical genus _Astacus_ reappears again on the other side of the Pacific, where several species occur in that part of North America which lies west of the Rocky Mountains. East of the Rockies, again, numerous species are found belonging to a distinct genus, _Cambarus_, which ranges from Canada to Central America and Cuba, and this genus is allied in certain respects to the _Cambaroides_ of Eastern Asia. If the systematic relations of these genera have been properly interpreted, it is by no means easy to understand in what way their present distribution has been brought about. [Illustration: _PLATE XX_ THE MURRAY RIVER "LOBSTER," _Astacopsis spinifer_. NEW SOUTH WALES. (MUCH REDUCED) THE LAND CRAYFISH, _Engæus cunicularis_. TASMANIA (NATURAL SIZE)] The Southern Crayfishes have an even more scattered and discontinuous range. In New Zealand the genus _Paranephrops_ occurs, in Australia and Tasmania the genera _Astacopsis_ (Plate XX.), _Cheraps_ and _Engæus_ (Plate XX.). A single species of _Cheraps_ has been recorded from New Guinea, but no Crayfishes are found in any part of the Malay Archipelago, in Southern Asia, or on the continent of Africa, although, curiously enough, a single species of a peculiar genus (_Astacoides_) is found in Madagascar. In South America species of _Parastacus_ are found in Southern Brazil, Argentina, and Chili. It is evident that these various genera of Parastacidæ, which are now so widely isolated from each other, must have reached their present habitats when the relative distribution of land and sea in the Southern Hemisphere was very different from what it is now. What exactly the nature of the land connection between the various islands and continents was, whether by way of an Antarctic continent or otherwise, is a question that can only be suggested here. To attempt to answer it would involve the consideration of the distribution of many other groups of animals besides Crayfishes.