THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY 



No. 133. OCTOBER 1847. 



XX. — Description of the Atlas, Axis, and Subvertebral Wedge 

 Bones in the Plesiosaurus, with remarks on the homologies of 

 those bones. By Prof. Owen, F.R.S. 



In my f Report on British Fossil Reptiles* ' two explanations are 

 offered of the special homologies of the subvertebral wedge-bones 

 discovered by Sir P. de M. Grey Egerton in the neck of differ- 

 ent species of Ichthyosaurus^, — the one as repetitions of the 

 ' odontoid process/ the other as of the so-called 'body of the 

 atlas ' in existing reptiles. Viewing the subvertebral wedge-bones 

 in their wider relations, I subsequently described them as " de- 

 tached developments of bone in the lower part of the capsule of 

 the notochord '* (chorda dorsalis, auct.) ; illustrating that view by 

 reference to the condition of the corresponding part of the ver- 

 tebral column in a large Siluroid fish J. Subjoined is a figure 

 of that remarkable structure (fig. 1) ; in which c o is the basi- 

 ng. 1. 



cS,ex 



c^er 



Section of anchylosed cervical or anterior abdominal vertebra? of Bagrus 

 tachypomus, nat. size. 



* Report of British Association, 8vo, 1839, pp. 100, 101. 

 t Geological Transactions, 2nd ser. vol. v. p. 187. pi. 14, 1836. 

 X Report on Vertebrate Skeleton, Rep. Brit. Assoc. 1846, p. 260. 

 Ann. fy Mag. N. Hist. Vol. xx. 16 



