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. [Illustration: _PLATE XXI_ _Palæmon jamaicensis_, A LARGE FRESHWATER PRAWN OF THE FAMILY PALÆMONIDÆ. WEST INDIES. (MUCH REDUCED)] 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. [Illustration: _PLATE XXII_ _Atya scabra_, A FRESH-WATER PRAWN OF THE FAMILY ATYIDÆ, WEST INDIES (REDUCED)]