A new species of myodocopid Ostracoda, Tetragonodon rex, is described and illustrated, including all growth stages. Based on morphology only, it is concluded that the adult male is capable of swimming, whereas adult females and juveniles are restricted to the sediment. Gut contents were unidentifiable, except for some foraminiferans. The species has five growth stages, and order of appearance of appendages, as well as their progressive morphological development, are similar to those of other Myodocopina in the subfamily Pseudophilomedinae. Methods for the determination of age of growth stages are discussed, and it is concluded that T. rex does not undergo postadult molting. Evidence for postadult molting in other myodocopid species is reviewed, and it is concluded that the question remains tenuously open for Macrocypridina and Gigantocypris. The production of eggs and mortality in the Myodocopina is discussed, and additional data is reported supporting clutch size being in part a function of adult size. In general, for those few species for which all instars are known, the Cypridinidae appear to have six stages, the Sarsiellidae and Rutidermatidae have five stages, the Philomedinae have six stages, and the Pseudophilomedinae have five stages. Within the Cylindroleberididae, the Asteropteroninae have five to seven stages, the Cylindroleberidinae have six stages, and the Cyclasteropinae have seven stages. Analysis of the relative lengths of adult males and females of the Philomedidae indicates that length differences are not affected by whether or not the adult female is capable of swimming.
