Paleothyris acadiana n.gen.,n.sp. is described from the Middle Pennsylvanian Morien Group of Florence, Nova Scotia. Two nearly complete skeletons and numerous disarticulated specimens were found in a single upright lycopod stump. On the basis of this genus, the phylogeny of the family Romeriidae is considered and the relationship of this family to other primitive reptiles is discussed. It is concluded that Araeoscelis should be classified among the captorhinomorphs, rather than as an euryapsid, that diadectids can be considered aberrant offshoots of the same stock which gave rise to typical primitive reptiles, that mesosaurs arose from early captorhinomorphs, and that pelycosaurs and romeriids share a common ancestry above the level of the limnoscelids.