The genera of primitive and synapsid reptiles which form the usual morphotypic series leading to cynodonts are not members of a phyletic series. Ecological analyses show three developing evolutionary lines during the Permo-Carboniferous—one lowland, living on deltas and in swamps; a second somewhat more upland; and a third distinctly upland. The members of these three lines are the source of the morphotypic genera, but most of them come from deposits formed in lowlands where they came by successive invasions from moreupland habitats. Evidence of the upland lines comes from these lowland sites, from invading animals, and from those introduced by mechanical transport from their upland habitats.

The brain cases and masticatory structures ofthe members of the morphotypic series—Hylonomus, Haptodus, Varanosaurus, Ophiacodon, Dimetrodon, Eotitanosuchus, Scymnognathus, Lycosuchus, Thrinaxodon—are examined in light of the ecological interpretations. Deformed coordinates applied to the lateral aspects of the skulls of the genera show clearly that the morphotypes do not provide a coherent evolutionary series. The “trends” of evolutionary change from Hylonomus to Thrinaxodon can best be seen if these two and Haptodus are used as an evolutionary series, the stages that are missing interpolated, and the roles ofthe other genera evaluated on the basis of this more or less idealized, conceptual series.