224 Prof. Owen on the Atlas, Axis, and Subvert ebral 



neck of the Cyclodus gigas gives some colour to the view of the 

 odontoid as the rudiment of a vertebra distinct from both atlas 

 and axis, and which in the Cyclodus is represented by the cen- 

 trum and hsemal spine, without the neural arch. 



The structure of the atlas and axis in the Crocodile (fig. 6) gives 

 further colour to this view. The odontoid piece (c a) is wholly 

 interposed between the wedge-shaped F - 6> 



cortical part of the body of the atlas, 



c a, e x, and the body of the axis, c x : jLT"*** — ^^3 



moreover the wedge-bone, c a, ex, not Wt*^ — >? <> 



only supports neurapophyses, n a, but 3^^^^)% 



also pleurapophyses, pi a: and the /E^PpSsSm*. ex 



odontoid, ca, in like manner, besides .^^^^^Ir 



giving some support to the neurapo- ^^^ 



physes, n x, also supports, and that ex- <^^ 

 clusively, the pleurapophyses, pi x, or Atlas and axis of the 

 second pair of cervical ribs. Gavial. 



The true centrum of the axis c x supports no ribs, and ap- 

 pears like an enormous epiphysis to c a, extended backwards to 

 aid in supporting the long neural arch n x. Neither the odontoid 

 c a, nor the wedge- shaped part of the atlas ca, ex, are produced 

 into inferior spines. If however, as the anatomy of the atlas 

 and axis in Lacertian Saurians has led me to conclude, the 

 odontoid c a is the homologue of the anterior (c a, figs. 2 & 3) 

 of the two anchylosed vertebral centrums described as atlas and 

 axis in the Enaliosaurs, the bifurcate pleurapophysis pi in the 

 Crocodile should be the displaced homologue of that which is 

 articulated to the posterior of those vertebral centrums in the 

 Ichthyosaurus, and the articular surface of which is shown at 

 p, c x, A, fig. 3, in the Plesiosaurus, and by Sir P. Egerton in 

 pi. 14. fig. 2. B d of his Memoir above-cited, in the Ichthyosaurus. 

 Whether the anterior vertebra c a, fig. 2, may also have supported 

 by the surface p, a rib, homologous with that of the atlas of the 

 Crocodile pi a, is uncertain ; but, if so, the atlantal rib in the 

 Crocodile would show, in like manner, a displacement forwards, 

 from the central part of the body c a, represented by the odon- 

 toid piece in recent Saurians, to the first subvertebral wedge- 

 bone ca,ex, which represents the body of the atlas in such Sau- 

 rians, and takes a share in the support of the neural arch, which 

 its homologue does not do in the Ichthyosaurus and Plesiosaurus. 

 If, however, the hypothesis that c a, fig. 6, in the Crocodile is the 

 homologue of the anchylosed atlas c a, figs. 2 & 3, of the Ena- 

 liosaurs, and that ca, ex, fig. 6, is the homologue of the first 

 wedge-bone, ca, ex, fig. 2, be saved by assuming an advanced 

 displacement of the pleurapophyses pi x and pi a, in the Croco- 

 dile, the numbers and relations of the inferior spines in the great 



