able rule among the Cotylosauria (Fig. 86 e) and occurs occasionally in the Theromorpha and even in the recent Sphenodon. Such ribs, though usually called single-headed, are not really so since both capitulum and tuberculum are present, though connected. A better name for them is holocephalous. Soon, however, the articular surfaces become restricted to the head and tubercle, that is, there is an emargination of the articular surface between them, the so-called neck, and the rib is truly double-headed, or dichocephalous (Fig. 86 d). Strictly speaking, single-headed ribs are those which have lost either the head or the tubercle.
Fig. 87. Dorsal vertebra of Thalattosaurus. (After Merriam.)
This early mode of articulation of double-headed ribs, the head across the intervertebral cartilage, the tubercle to the diapophysis of the arch, has continued through those reptiles [see above] and through the mammals. And this is essentially the mode of rib articulation in the Diaptosauria.
In many reptiles, however, perhaps in part because of the closer articulation of the vertebrae, the head has migrated backward to articulate with a facet or process on the anterior end of the centrum, the parapophysis, and there it has remained in the cervical vertebrae of most reptiles, and in the dorsal vertebrae of the Squamata and their allies. In the dorsal region there have been many modifications. In those reptiles which are here classed under the Parapsida, that is, the Ichthyosauria, Proganosauria, Pleurosaurus, and Squamata (Figs. 80 n, 73 c–f), the tubercular part of the articulation has been largely or wholly lost, and the single-headed ribs remained attached more or less wholly to the centrum. In the later Ichthyosauria and later Plesiosauria, it is true, the ribs are often dichocephalous (Fig. 86 c), both articulations uniting with the centrum. There is, however, in such forms no real tubercle. The ribs of Araeoscelis, a Lower Permian reptile, are single-headed and central in the cervical region, imperfectly double-headed in the dorsal region. So also, the ribs are described as single-headed in Pleurosaurus, Protorosaurus, the Proganosauria, and Thalattosauria (Fig. 87), probably all with a single, typical, upper temporal opening. The dorsal ribs (Figs. 80 o, 89) of the
