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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.

Before leaving the Crayfishes, it may be mentioned that certain species have become adapted to almost terrestrial habits. A number of species of Cambarus in North America are often found at considerable distances from open water, burrowing in damp earth, their burrows reaching down to the ground-water. In many cases they throw up chimney-like piles of mud at the mouths of their burrows, and in places their chimneys are so numerous as to "hamper farming operations by interfering with the harvesting machines, clogging and ruining them." The species of Engæus ([Plate XX].), found in Tasmania, are there known as "Land Crabs," and burrow in marshy places and in the forests up to an elevation of 4,000 feet.

PLATE XXI

Palæmon jamaicensis, A LARGE FRESHWATER PRAWN OF THE FAMILY PALÆMONIDÆ. WEST INDIES. (MUCH REDUCED)

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The broad equatorial belt which separates the regions inhabited by the Northern and the Southern Crayfishes is characterized by the presence of several other groups of fresh-water Decapoda. The large River Prawns, which are found nearly everywhere within the tropics, belong to the genus Palæmon ([Plate XXI].), which is very closely related to the common marine Prawns (Leander) of our own coasts. Some of these Prawns grow to a foot or more in length of body, and the large claws may measure as much again. From the Crayfishes, for which they are sometimes mistaken, they may be easily distinguished by the fact that the large pincer-claws are not the first, but the second pair of legs. Another widely-spread group of River Prawns, for the most part of small size, is the family Atyidæ ([Plate XXII].), in which the two pairs of pincer-claws are feeble, and have the fingers tipped with brushes of long hairs, used in sweeping up minute particles of food from the mud. The distribution of these Prawns presents many difficult problems, as an example of which we may mention the presence of identical or closely related species in the fresh waters of West Africa and of the West Indies.