Reptiles of several distinct lineages have been described from the lower part of the Upper Carboniferous. Neither reptiles nor any plausible ancestors are known from earlier beds. The nature of the amphibian-reptilian transition must be studied on the basis of relicts of earlier groups which are contemporary with true reptiles. Several genera from the Middle Pennsylvanian locality of Nyrany, Czechoslovakia, have been considered closely related to the ancestry of reptiles. The incomplete nature of the original descriptions of Diplovertebron, Gephyrostegus and Solenodonsaurus has led to considerable confusion as to their taxonomic distinction and phylogenetic significance. Subsequently these taxa have been variously synonymized and considered as being alternately embolomeres, primitive anthracosaurs, seymouriamorphs, extremely primitive reptiles and captorhinomorphs. On the basis of redescription of type material from museums in Berlin, Prague and Cambridge as well as more recently discovered specimens, it is possible to establish the identity of these forms and to describe them in considerable detail. The type of Diplovertebron punctatum can be identified as a small embolomere. The material on which it is based can be distinguished from the types of Solenodonsaurus and Gephyrostegus. The type of Gephyrostegus bohemicus is a primitive anthracosaur. On the basis of this and other material, description of almost the entire skeleton is possible. The vertebral column has 24 presacral segments, each consisting of a large U-shaped pleurocentrum and a smaller, crescentic intercentrum. This genus is an almost ideal relict of the anthracosaur group which gave rise to reptiles. It is clearly distinct from the Permian seymouriamorphs, and could not have given rise to any known members of that group. Eusauropleura digitata from a deposit of equivalent age at Linton, Ohio, is a related, but somewhat more primitive from. The body proportions of these genera indicate that they were terrestrial in habit, with no specifically aquatic adaptations. The type of Solenodonsaurus janenschi is a very primitive reptile. This form retains an otic notch and labyrinthine infolding of the enamel, but the postcranial skeleton is very close to the pattern of romeriid captorhinomorphs. Material of a very immature specimen assigned to this genus by Pearson has a typically reptilian palate and skull roof pattern, but anthracosaurian dorsal and ventral scales. Material (designated by them as specimen I) assigned to the genus Gephyrostegus by Brough & Brough actually pertains to a romeriid captorhinomorph. On the basis of this material, the ancestry of captorhinomorph reptiles from the gephyrostegid anthracosaurs can be firmly established. The families Solenodonsauridae and Limnoscelidae are relicts of more primitive, but still reptilian, lineages which retain a number of anthracosaurian characteristics.