144 Prof. H. G. Seeley on 



innermost layer is formed of superposed delicate lamellte. A 

 transverse section of the spine in the British Museum 

 examined microscopically exhibits neither bone-cells nor 

 distinctly recognizable vascular dentine. The small vascular 

 canals are surrounded by concentric lamellge of seemingly 

 structureless tissue ; and the only appearances suggestive of 

 the canaliculi of vascular dentine are observed in the series of 

 prominences which are thrust from the central core into the 

 distinctly separable outermost layer of the spine. There is 

 no thick zone of lamella? concentric with the median cavity, 

 such as is described by E-ohon in Onchus * ; but some are con- 

 centric with the wavy border of the peripheral layer already 

 mentioned, and it is unfortunate that the precise nature of the 

 latter cannot be discerned in the section examined. 



Byssacanthus is thus proved to be not an ordinary Elasmo- 

 brancli spine; and it is extremely probable that the fossil 

 belongs to a totally distinct group. At present the writer 

 would compare it with the spinous plate of the Ostracoderm 

 Ceraspis'\, though this is distinguished by its remarkable 

 thickness of coarsely cancellated tissue, and we as yet have 

 no intbrmation concerning its microscopical structure. Among 

 known Ostracodermi the histology of Byssacanthus is most 

 nearly paralleled by that of the Pteraspidians ; and the recog- 

 nition of the other elements which must have entered into the 

 same armature as the spinous plate will be awaited with 

 interest. 



XVII. — On Thecodontosaurus and Palaiosaurus. 

 By H. G. Seeley, F.R.S.+ 



The well-known memoir by Dr. Henry Riley and Mr. Samuel 

 Stutchbury on three distinct Saurian animals discovered in 

 18o4 in the Magnesian Conglomerate on Durdham Down, 

 near Bristol, was communicated to the Geological Society in 

 18.'56, and published in the Transactions of the Society in 

 1840. Those fossil animals from the Trias became known 

 as Thecodontosaurus antiq^uus (Trans. Geol. Soc. 2nd ser. 



* J. V. Rohon, " Die Obersilurischen Fisclie vou Oesel," Mem. Acad. 

 Imp. Sci. St. Petersbourg, [7] vol. xli. no. 5 (1893), pp. 41-46, pi. ii. 

 fig. 58. 



t C. Schliiter, Sitzungsb. niederrhein. Gesell. Bonn, 1887, p. 120; A. S. 

 Woodward, Catal. Foss. Fishes Brit. Mus. pt. ii. (1891) p. 233. 



\ Read before the Geological Society of London, June 22, 1892, as 

 Part 5 of " Contributions to Knowledge of the Saurischia of Europe and 

 Africa." 



