The nature of the vertebrae has long been used to define the major taxa within the Labyrinthodontia and to assign a place to the group within the classification of the Amphibia. The most diagnostic feature is the mode of ossification of the region of the centrum. Early labyrinthodonts had the centrum ossified in two principal and separate regions, an intercentrum (hypocentrum), often an anteroventral crescentic wedge, and a pleurocentrum, lying posterior and often dorsal to the intercentrum. It is now fairly generally agreed that the rhachitomous vertebra represents the primitive labyrinthodont condition (Romer, 1947). In this type a wedge-shaped intercentrum lies in front of laterally paired, dorsal, pleurocentral blocks (e.g. Eryops: Fig. 4B). However, for many years there was little evidence of rhachitomous vertebrae below the Upper Carboniferous and Watson (1912, 1919, 1926, 1929) regarded the embolomerous vertebra, typically associated with the Coal Measure anthracosaurs, as the most primitive type. Description of the vertebrae in some Rhipidistia, the fish group ancestral to the tetrapods, and in the earliest known labyrinthodonts, the Ichthyostegalia (Jarvik, 1952), gives added support to the contention that the rhachitomous vertebra is primitive. Both rhipidistians, such as Eusthenopteron from the Upper Devonian, and the ichthyostegids from the Devonian-Carboniferous boundary had a typical wedgeshaped intercentrum and paired posterodorsal blocks, which may be homologous with the rhachitomous pleurocentrum (Fig. 1, A and B). Romer's views are further confirmed by the report of rhachitomus vertebrae articulated with a skull of the loxommid Megaloce phalus (Baird. 1957). reDresenting a group placed by Watson, in the absence of associated vertebrae, within the Embolomeri. Vertebrae little removed from the rhachitomous type have also recently been described by Romer (1964) in the Lower Carboniferous Pholidogaster, probably to be regarded as a very primitive anthracosaur (Fig. 1C). Romer's (1947) scheme for the evolution of the various types of vertebrae found within the labyrinthodonts is well known and is reproduced in Figure 2. To the types illustrated must be added the unique and aberrant pattern found in the plagiosaurs (Panchen, 1959), whose derivation is quite uncertain (Fig. 6). For many years the theory of the morphology of vertebrae originally proposed by Gadow and Abbott (1895) colored the thinking of all workers in the field and was taken as a point of departure for consideration of vertebral structure in the labyrinthodonts. The theory of Gadow and Abbott, which was extended to apply to the tetrapods by Gadow in 1896, was based on the now untenable assumption that modern elasmobranchs showed the basic scheme for all craniate vertebrae, and specifically that the structure of tetrapod vertebrae could be derived by reductions or loss of elements of the elasmobranch pattern. Like many persistent but erroneous theories, Gadow's proposed a nomenclature which was widely adopted. He assumed that all vertebrae were formed from four pairs of symmetrically arranged cartilaginous elements, termed arcualia, which he named individually. Embryological study of the origin of vertebrae in living Amphibia (see references in Williams, 1959) made it clear that Gadow's theory was of little help here. The ossification of the centrum was perichordal and in