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Show 1903.] ON THE VERTEBRAE OF THERIODONTS. 177 3. On the Axis, Atlas, and Proatlas in tlie Higher Theriodonts. By H. Broom, M.D., B.Sc., C.M.Z.S. [Received January 20, 1903.J (Plate X V II I .1) Hitherto, though something has been known of the structure of the axis in at least two of the Dicynodonts and in Cynognathus, we have been practically ignorant of the structure of the atlas in any of the Theriodonts or Anomodonts. In fact, Prof. Seeley (1) in describing the axis of Cynognathus, seems to be in doubt whether he is really dealing with an axis, or with an axis and atlas combined. On page 100 of his paper he says, " The first vertebra appears to be anchylosed to the second " ; while a little further on he states that " this vertebra is remarkable for its form being exactly like the odontoid process of the vertebra in many animals, and suggesting the idea that the atlas is lost " ; and in a note he adds : " This appears to be confirmed by the subsequently to be described condition in Tropidostoma dunni" In counting the vertebra, however, he counts the axis as the 1st cervical, and states that " there are 6 cervicals." Gadow (2), in his recently published work on Reptiles, possibly misled by Seeley, definitely states that " the atlas is fused with the axis." Considering how mammal-like the higher Theriodonts are, and that the axis bears a very marked resemblance to the mammalian axis, having a large spine and a well-developed odontoid process, it seems remarkable that the idea should have arisen that the atlas was anchylosed to the axis, more especially as there is no part of the axis in Cynognathus that bears any resemblance to any known atlas. When in Grahamstown recently, I had an opportunity, through the kindness of Dr. Schonland, of examining the Theriodonts in the Albany Museum, and especially the very fine specimens of Gomphognathus and Trirachodon, which have been described by Seeley (3), and which have been so marvellously developed under his direction. In both the type specimens of Gomphognathus kannemeyeri and Trirachodon kannemeyeri the anterior cervical vertebra? are most beautifully shown, but Seeley in his description of the specimens makes no reference to this most important region, beyond stating (p. 53) that " the occipital plate [in Trirachodon] is not completely exposed owing to some of the vertebrae beino- in contact." Two of his figures of the skull of Trirachodon show the atlas and part of the axis ; but as no distinction is shown between matrix and bone, it is difficult to make anything of the ^ T h e beautiful specimen which forms the type of Gomphognathus i For explanation of the Plate, see p. 180. P r o o . Z ool. Soc.- 190.>, V o l . I. No. X II. 12 |