PROF. OWE>~ ON SOAIE DIXOSA.T7RIAX VERTEBRA. -±J 



terior one, receiving the single .occipital condyle by the somewhat 

 deeper anterior concavity, and affording the subtriangular rough 

 surface (figs. 4 & 5, hij) for the attachment of an atlanto-occipital 

 "wedge-bone:" the smaller hinder surface (fig. 5, hy') indicates 

 a second smaller "wedge-bone " in the neck of Pareiasaurus. After 

 securing five views of this vertebra, I made a longitudinal trans- 

 verse section of the centrum, and, with the finely cancellous texture, 

 exposed the pair of conical pits (fig 7, c', c'). The initial canal is 

 feebly marked ; the excavation is chiefly conical, the cones wider 

 and shorter than in the succeeding vertebras, and their apices se- 

 parated by osseous tissue of a line in breadth. 



In the above described and figured nnossified tracts of the middle 

 of the centrum in Tapinocephalus and Pareiasaurus, we have indica- 

 tions of a persistent trace of the primitive " chorda dorsalis." In 

 the larger Dinosaur a beaded remnant would seem to traverse some 

 proportion of the vertebral column, as in certain fishes*. It is a 

 trace of a lower grade of vertebral structure, or stage of develop- 

 ment of the centrums, and is interesting as being associated with a 

 Dinosaurian type of probably Triassic age. 



It recalls the more widely perforated, still less ossified, centrums 

 of the vertebras of Ganocephalous reptiles of the Carboniferous 

 series, represented by Parabatrachus, Hylonomus, Dendrerpeton, &c, 

 and thus strengthens the evidence of the post-Liassic age of the 

 Karroo series given in the Quart. Journ. GeoL Soc. vol. xxiii. 

 p s 140. 



Tor the family of Dinosauria, represented by the genera Tapino- 

 cephalus and Pareiasaurus, I have proposed the name Tretospon- 

 dylia. 



In conclusion, I have to express my deep obligations to the late 

 Andrew Geddes Bain, F.G.S., and to my friend Dr. Guibon Ather- 

 stone, F.G.S., for the fossil materials from Blinkwater and Gats 

 Plaatz, South Africa, from which have been derived the above- 

 described structures. 



EXPLANATION OF THE PLATES. 

 Plate IT. 



Fig. 1 . Anterier view of centrum, and anchylosed base of neural arch of a dorsal 

 vertebra, of Tapi'/iocephalus Atherstonii, Ow. 

 la. Outline of section and plane of the articular surface of the centrum. 

 2. Side view of the same vertebra. 



Plate V. 



Fig. 3. Longitudinal and vertical section of a dorsal centrum of Pareiasaurus 

 serridens. 



4. Anterior view of centrum, and anchylosed base of neural arch, of a cer- 



vical vertebra of Pareiasaurus bombidcns, Ow. 



5. Side view of the same vertebra. 



6. Upper view of the same vertebra. 



7. Longitudinal and transverse section of the centrum of the same ver- 



tebra. (The dotted lines show the lay of the surface as wrought out 

 prior to the section.) 



(All the figures are of the natural size.) 



* Scymnus, Mugil, e. g., 'Atiat. of Vertebrates,' 8vo, vol. i. pp. 32, 37, fig, 31. 



