The Atlas-Axis Complex

In lizards, the atlas consists of the first intercentrum (h₁) which is fused with or sutured to the two half neural arches. It thus forms a ring which supports the ventral and lateral edges of the occipital condyle. The odontoid forms an apex dentis which comes to lie within a trough on the dorsal surface of the occipital condyle. The second intercentrum (h₂) is fixed below the centrum of the axis, and constitutes the anterior hypapophysis which is generally directed posteriorly. In the Anguinomorpha, the third intercentrum (h₃) is fused with or sutured to the posterior part of the proceeding centrum, i. e. the axis, which consequently bears two hypapophyses. These may sometimes fuse into a single crest (Hoffstetter and Gasc, 1969). The discussion of the anguinomorph atlas-axis complex will concentrate on the most conspicuous features only. To take the details of the structures into account seems not safe enough without previous analysis of possible individual variation which may be very extensive. For example, the h₂ and h₃ on the axis were found to have fused into a continuous crest in one specimen of Ophisaurus apodus (Fig. 26, c), but they remained separate in a second specimen.