| OCR Text |
Show 1903.] VERTEBRAE OF THERIODONTS. 179 arch and hypapophysis of the atlas, and the odontoid process and part of the body of the axis. Though the structure of the vertebras is essentially similar to that in Gomphognathus there are many points of difference. The proatlas (p.a.), though occupying the same relative position as in Gomphognathus, is less specialized. It is present as a pair of curved, moderately thin, bony plates lying in front of, and probably somewhat overlapping, the arch of the atlas. There is 110 bony process developed as in Gomphognathus. The atlas (at.) consists of a well-developed arch and a distinct hypapophysis. The arch is not so complete as in Gomphognathus, and must have differed considerably in regard to the arrangement of the anterior articular surfaces. In Gomphognathus the occipital condyles (o.c.) are small and moderately close together, whereas in Trirachodon the condyles (o.c.) are very large and as wide as the atlas. The arch, perhaps on this account, does not close in inferiorly to meet the hypapophysis, with which it can only have been attached by ligament. The zygapophysis (z.at.) for the axis is very small. The hypapophysis (h.at.) is very similar to that in Gomphognathus. Of the axis only the odontoid process, imperfectly displayed, and a part of the body remain. The structure of the upper cervical vertebrae is so imperfectly known in the majority of fossil reptiles, that there is some difficulty in satisfactorily dealing with the affinities of the structures. It is difficult to avoid being struck by the close resemblance of the axis, atlas, and proatlas in the Tlieriodont to those structures in the Crocodiles, more especially as there is no close affinity between the groups. The explanation is probably to be found in the fact that both the Crocodiles and the Theriodonts, though far removed from each other, have retained with but slight modification the type of structures met with in their common ancestor. In Sphenodon, though the proatlas is small, the type is practically the same ; and I have recently discovered that Procolophon has a well-developed proatlas. Procolophon has usually hitherto been associated with the Dicynodonts, Theriodonts, and Pareiasaurians, but as it has a persistent notochord, abdominal ribs, a plate-like pubis and ischium, and 2, 3, 4, 5, and 4 phalanges in the digits, it is manifestly much more closely related to Palceohatteria than it is to the Theriodonts and Dicynodonts. In most of its structures it is probably as primitive as the common ancestor of the Crocodiles and Theriodonts, so that there is good reason for believing that the common ancestor had a well-developed proatlas and probably an atlas and axis very similar to those found in later forms, but almost certainly with the odontoid process as a distinct element, as seen in Ichthyosaurus. No proatlas has yet been detected in Pareiasaurus, but I have found one in Lystrosaurus ( = Ptychognathus Owen). It will probably yet be found in most of the primitive reptilian types. |
