This paper presents an integrated quantitative study of the ontogeny and phylogeny of species of the ostracode genus Polytylites based on about 1,600 specimens from the Chester Series of southern Illinois and western Kentucky. Examples of phyletic size increase, proterogenesis, and paedomorphosis are described; phyletic growth, in this example, probably resulted from specific evolutionary pressures rather than from the general selective advantages usually presumed to accrue to larger size. An empirical scheme of morphologic comparison is developed to establish intrageneric relationships. Evidence suggests that variability of growth factors is due to changing allometric patterns rather than to overall retardations and accelerations of growth. The taxonomy of the group is reviewed, and one new species, Polytylites websteri, is erected.