144 Prof. H. G. Seeley oji 



innermost layer is formed of superposed delicate lamelhe. A 

 transverse section of the spine in the British Museum 

 examined microsco})ical]y exhibits neither bone-cells nor 

 distinctly recognizable vascular dentine. The small vascular 

 canals are surrounded by concentric lamellie of seemingly 

 stiuctureless tissue; and the only appearances suggestive of 

 the canaliculi of vascular dentine are observed in the series of 

 prominences \\hicli are thrust from the central core into the 

 distinctly seiiarable outermost layer of the spine. There is 

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

 such as is described by Kohon 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- 

 branch 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 Ostracoderra 

 Ceraspis'\y though this is distinguished by its remarkable 

 thickness of coarsely cancellated tissue, and we as yet have 

 no information 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 Palseosaurus. 

 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 

 1834 in the Magnesian Conglomerate on Durdham Down, 

 near Bristol, was communicated to the Geological Society in 

 1836, and published in the Transactions of the Society in 

 1840. Those fossil animals from the Trias became known 

 as Thecodontosaurus antiqaus (Trans. Geol. Soc. 2nd ser. 



* J. V. EohoD, " Die Obersiluriscben Fische von Oesel," ^lem. Acad. 

 Imp. Sci. St. Petersbourg, [7] vol. xli. no. o (1893;, jip. 41-45, pi. ii. 

 fig. 58. 



+ C. Scbliiter, Sitzungsb. niederrhein. Gesell. Bonn, 1887, p. 120 ; A. S. 

 Woodward, Catal. Foss. Fisbes-Brit. Mu3. pt. ii. (1891) p. 233. 



X Read before tbe Geolo-jical Society of London, June 22, 1892, as 

 Part o of " Contributions to Knowledge of the Sauriscbia of Europe and 

 Africa." 



