Of the Decapoda, seven families are typically fresh-water in habitat—the Aegleidae, containing the single species _Aeglea laevis_, related to the Galatheidae, which inhabits streams in temperate S. America; the Atyidae, a family of Prawns from the tropical rivers and lakes of the New and Old World, and in the Mediterranean region. A number of Palaemonidae are found in fresh water, e.g. _Palaemonetes varians_ in Europe and N. America, while several species of _Palaemon_ occur in lakes, streams, and estuaries of the tropical Old and New World. The expeditions of Moore and Cunnington to Lake Tanganyika brought back an exceedingly rich collection of Prawns, comprising twelve species, all of which are peculiar to the lake,[173] and this is all the more surprising since Lakes Nyasa and Victoria Nyanza are only known to contain one species, _Caridina nilotica_, which ranges all over Africa and into Queensland and New Caledonia. The Tanganyika species, however, all belong to purely fresh-water genera, and do not afford any suggestion that they are part of a relict marine fauna. It would appear that they have been differentiated in the lake itself during a long period of isolation. Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a sub-family of the Grapsidae), are fresh-water. _Thelphusa fluviatilis_ is an inhabitant of North Africa, and penetrates into the temperate regions of the Mediterranean, and is said to be exceedingly common in the Alban Lake near Rome. Both these families have representatives on land, e.g. _Potamocarcinus_ in Central and South America, and certain species of _Sesarma_, and the closely related Gecarcinidae of the West Indies.