Wedge Bones in the Plesiosaurus. 225 



Cyclodus do not lend themselves so readily to the same determi- 

 nation of the odontoid in that lizard, as a part or complement of 

 the body of the first vertebra. 



The number of the inferior spines might be made to corre- 

 spond with that of the vertebrae by supposing either o d, ex, or 

 c x, ex, in fig. 5, to be an accessory exogenous process, and not 

 a true homotype of the other four spines ; or the spine ex, ex, 

 may be held to belong properly to the third vertebra, and with 

 the succeeding spines to be abnormally advanced and anchylosed 

 to the vertebra anterior to the one to which it properly belongs. 

 Yet both these suppositions appear to be equally arbitrary. The 

 condition of the odontoid and axis in the Cyclodus is, neverthe- 

 less, an exceptional one in the Lacertia, and I no longer regard 

 the distinct inferior spine ex, ex, in fig. 5, as a proof that the 

 odontoid, like the atlas, is the homologue of one of the sub verte- 

 bral wedge-bones. I still retain the opinion that it is not "the 

 peculiarly developed anterior articular epiphysis of the second 

 vertebra"*; but I return to my former idea of the special ho- 

 mology of the odontoid piece in Saurians, and consequently the 

 odontoid process in mammals, with the part called the anchy- 

 losed atlas in the Ichthyosaurus ; and the subsequently ascer- 

 tained structure of the parts in the Plesiosaurus has confirmed 

 the conclusion that the first subvertebral wedge-bone in the 

 Ichthyosaurus and Plesiosaurus represents the part which has 

 been called ' body of the atlas ' in existing reptiles, but is reduced 

 to a still more atrophied condition than in themf. 



With respect to the general homology of these parts, the first 

 wedge-bone is a detached part of the cortex of the body of the 

 atlas, and the so-called atlas in the Enaliosauria or the odontoid 

 piece in existing Sauria is the central ossification of the same 

 vertebral element. The anchylosis of the atlas and axis is no 

 longer, therefore, a peculiarity of the Enaliosauria, but a struc- 

 ture essentially repeated in every higher vertebrate form up to 

 Man, in whom the anchylosed part of the atlas bears the anthro- 

 potomical name of ' odontoid process/ It might be expected that 

 the segment immediately succeeding those of the skull would be 

 the seat of more extensive and remarkable modifications than the 

 succeeding vertebrae of the trunk, and each modification will be 

 found, as the habits and mode of life of the different species be- 

 come better known, to be expressly adapted to such habits. But 

 such recognition of final causes by no means precludes the neces- 

 sity for every legitimate attempt to uplift the veil which hides 

 the type upon which all the adaptive modifications of the endo- 

 skeleton are based. 



* Report on Archetype of Vertebral Skeleton, 1846, p. 261. 

 f Report on British Fossil Reptiles, 1839, p. 101. 



