I02 THE OSTEOLOGY OF THE REPTILES 



centrum of the proatlas is the so-called intercentrum of the atlas, 

 necessitating the view that the axial intercentrum is merely an ac- 

 cessory or provisional bone developed below the odontoid to fill out 

 what would otherwise be an unoccupied space! 



Positive evidence of the proatlas has been discovered in several 

 genera of the Cotylosauria, but no complete specimen has yet been 

 discovered ; it is doubtless present throughout the order. It is present 

 in many if not all forms of the Theromorpha and Therapsida. In 

 Ophiacodon (Fig. 78) and Dimetrodon (Fig. 79) of the former group, it 

 is a small bone on each side, articulating in front by a facet on the 

 exoccipital, behind with an anterior zygapophysis on the arch of the 

 atlas, both surfaces looking more or less downward. These articular 

 surfaces appear to be present in all known genera. In the Crocodilia, 

 occurring as far back as Jurassic times, it is a single bone in the adult, 

 roof-shaped, arising from paired cartilages. In Iguanodon (Fig. 80 l), 

 of the predentate dinosaurs, as also in several genera of the Sauro- 

 poda, and the Triassic Plateosaurus of the Theropoda, it is paired, as 

 in the modern Sphenodon (Fig. 80 d) , also articulating with the atlas. 

 A roof-shaped, unpaired proatlas has been described in Rhampho- 

 rhynchus, a Jurassic pterosaur. It has also been reported in the cha- 

 meleon lizards and the mammals Erinaceus and Macacus. As an 

 abnormal element it was also found by Baur in a trionychoid turtle 

 {Platypeltis spinifer, Fig. 32), partially fused with the occiput, and 

 articulating with the arch of the atlas in the primitive way, from 

 which he concluded that the real body of this vertebra had become 

 permanently fused with the basioccipital. Probably it will be even- 

 tually discovered in many other extinct reptiles. 



Atlas (Figs. 78, 79, 80). There is no vertebra in the known amphib- 

 ians which can be homologized with the atlas of reptiles. By some 

 the so-called atlas of the amphibians is thought to be represented by 

 the proatlas; or it may have entirely disappeared. In the earliest 

 reptiles (Fig. 79), the atlas is temnospondylous in structure, that is, 

 composed of a paired arch resting in part upon a large, wedge- 

 shaped intercentrum, in part upon a single large, embolomerous, 

 notochordal pleurocentrum, all of them loosely connected with the 

 axis, the arch of the atlas or neurocentrum articulating in the usual 

 way by zygapophyses. 



In its highest development, in the mammals, the arch and inter- 



