Geographically, the Phyllopoda are cosmopolitan, representatives of every family and of some genera (e.g. _Streptocephalus_, _Lepidurus_, _Estheria_) being found in every one of the great zoological regions, though a few aberrant genera are of limited range, thus _Polyartemia_ is known only from the northern Palaearctic and Nearctic regions, _Thamnocephalus_ only from the Central United States. The genus _Artemia_ is not at present known in Australia.[23] The only recorded British species are _Chirocephalus diaphanus_, _Artemia salina_, and _Apus cancriformis_,[24] but other continental islands, for example the West Indian group, are better supplied. The distribution of the species is very imperfectly known, but on the whole every main zoological region seems to have its own peculiar species, which do not pass beyond its boundaries. _Branchinecta paludosa_ and _Lepidurus glacialis_ are circumpolar, both occurring in Norway, in Lapland, in Greenland, and in Arctic North America; but with these exceptions the Palaearctic and Nearctic species seem to be distinct. The European species _Apus cancriformis_ occurs in Algiers, but the relations between the species of Northern Africa as a whole and those of Southern Europe on the one hand, or of Central and Southern Africa on the other, have yet to be worked out. The soft-bodied Branchipodidae are not known in the fossil condition;[25] an _Apus_, closely related to the modern _A. cancriformis_, has been found in the Trias, but the most numerous remains have been left, as might be expected, by the hard-shelled Limnadiidae; carapaces, closely resembling those of the modern _Estheria_, are known in beds of all ages from the Devonian period to recent times; these carapaces are in several cases associated with fossils of an apparently marine type. None of the fossil species differ in any important characters from those now living, so that the Phyllopoda have existed in practically their present form for an enormously long period; this fact, and the evidence that species of existing genera were at one time marine, explain the wide distribution of animals at present restricted to a remarkably limited range of environmental conditions. =Summary of the Characters of the Genera.= SUB-ORDER PHYLLOPODA.—Branchiopoda with an elongated body, provided with at least ten pairs of post-cephalic limbs, the heart extending through four or more thoracic segments, and having at least four pairs of ostia. =Fam. 1. Branchipodidae.=[26]—Carapace rudimentary, eyes stalked; the second antennae flat and unjointed in the female, jointed and prehensile in the male; female generative opening single; telson not laterally compressed, bearing two flattened lobes, or none. The heart extending through the thorax and the greater part of the abdomen. A. Eleven pairs of praegenital ambulatory limbs.