THE RIBS AND STERNUM 



113 



able rule among the Cotylosauria (Fig. 86 e) and occurs occasion- 

 ally 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 sur- 

 faces 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 dichocephaloiis (Fig. 

 86 d). Strictly speaking, single-headed ribs are those which have 

 lost either the head or the tubercle. 



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 mam- 

 mals. And this is essentially the mode of rib artic- 

 ulation 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 cen- 

 trum, the parapophysis, and there it has remained 

 in the cervical vertebrae of most reptiles, and in 

 the dorsal vertebrae of the Squamata and their fig. 87. Dorsal verte- 

 allies. In the dorsal region there have been many )Z°^ Thaiattosaurus. 



° _ -^ (After Merriam.) 



modifications. In those reptiles which are here 

 classed under the Parapsida, that is, the Ichthyosauria, Progano- 

 sauria, Pleurosaurus, and Squamata (Figs. 80 N, 73 c-f), the tuber- 

 cular 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 unit- 

 ing 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 



