The Phreatoicidae are a curious family of Isopods confined to the fresh waters of Australia and New Zealand, which bear a remarkable resemblance to Amphipods, being laterally compressed and possessing a subchelate hand on the anterior thoracic leg. These Isopods are exceedingly common in small mountain pools and in streams in Tasmania, and in the Great Lake in that country I have recently found a number of species which, together with some species of Amphipods, make up the dominant feature in the Crustacean fauna. One of these species may grow to fully an inch in length. The family is confined to the temperate regions, and is usually found on mountains. A number of species are known from the mainland of Australia, one coming from a high elevation on Mount Kosciusko, and another (_Phreatoicopsis_) from the forests of Gippsland attaining a great size, and living among damp leaves, etc. The fresh-water Amphipoda all belong to the families Talitridae, Gammaridae, and Haustoriidae (see p. 137). Among the Talitridae, or Sand-hoppers, _Orchestia_ and _Talitrus_ have marine as well as fresh-water and land representatives, while the American _Hyalella_ is entirely from fresh water, most of the species being peculiar to Lake Titicaca. Many of these animals are partly emancipated from an aquatic life. Thus _Orchestia gammarellus_, which is common on the sea-shore of the Mediterranean, frequently penetrates far inland, and was found in large numbers by Kotschy near a spring 4000 feet up on Mount Olympus. _Talitrus sylvaticus_ is very common among fallen leaves and decaying timber in Tasmania and Southern Australia, many miles from the sea, and often at an elevation of several thousand feet. Among the Gammaridae, certain genera, e.g. _Macrohectopus_ (_Constantia_), from Lake Baikal, are purely fresh-water. An enormous development of Gammaridae was discovered by Dybowsky in Lake Baikal, comprising 116 species, and lately a number more have been found by Korotneff.[172] The majority of these were originally placed in the genus _Gammarus_, but Stebbing has rightly created a number of peculiar genera for them. Certain species are, however, placed in more widely distributed genera, e.g. _Gammarus_ and _Carinogammarus_, which is also represented in the Caspian Sea. Korotneff found some remarkable transparent pelagic forms (_Constantia_) swimming in the abyssal regions at about 600 metres depth, the majority of them being blind, but some possessing rudimentary eyes, often on one side only. Besides various species of _Gammarus_, a number of other Gammaridae are frequently found in brackish water. Among Haustoriidae _Pontoporeia_ has representatives in both the oceans and inland lakes of the northern hemispheres (see p. 137).