The _PHYLLOPODA_, which belong to the Crustacea in the strict sense of the term, comprise the Estheriidae and Cladocera (water-fleas). The former group is represented by _Leaia mitchelli_, which is found in the Upper Carboniferous or Carbopermian of the Newcastle District, New South Wales. In the still later Hawkesbury series (Triassic) of New South Wales, _Estheria coghlani_ (Fig. 111 D) occurs. This species is a minute form, the carapace measuring from 1.25mm. to 2mm. in the longer diameter of the shell. In the upper part of the Wairoa Series (Triassic) of Nelson, New Zealand, there is found another species of _Estheria_, identified with a European form _E. minuta_. _Estheria mangaliensis_ is another form occurring in the Jurassic (Ipswich series) of Queensland. At the present day these little _Estheriae_ sometimes swarm in countless numbers in freshwater lakes or salt marshes. =Ostracoda: Their Structure.--= Passing on to the next group, the bivalved _OSTRACODA_, we note that these have existed from the earliest geological periods to the present day. They are usually of minute size, commonly about the sixteenth of an inch in length, although some attained a length of nearly one inch (_Leperditia_). Their bodies are indistinctly segmented, and are enclosed within a horny or calcareous shell. This shell consists of two valves which are joined along the back by a ligament or hinge, the ends and ventral edge remaining quite free. The pairs of appendages present are the antennae (2), mandibles (1), maxillae (2), and thoracic feet (2). The only portion found in the fossil state is the bivalved carapace, the two valves being frequently met with still united, especially when these tiny animals have settled down quietly on the sea-bed and have been quickly covered with sediment. =Features of the Ostracod Carapace.--= Since the body parts of the ostracod are wanting in the fossil examples, the generic determination is attended with some difficulty, especially in regard to the smooth or bean-shaped forms. The chief distinctive characters to note are, the contour of the carapace seen in three directions (top, side and end views), the structure of the hinge, and the position and figure of the muscle-spots or points of adhesion of the muscular bands which hold or relax the two valves. The valves in certain genera fit closely upon one another. In others, one overlaps the other, the larger being sometimes the right (as in _Leperditia_), sometimes the left (as in _Leperditella_). The hinge-line is often simple or flange-like, or it may consist of a groove and corresponding bar, or there may be a series of teeth and sockets. Lateral eye-tubercles are sometimes seen on the surface of the valve, whilst in the animal there was also a small eye. =Habits of Ostracoda.--=