222 Prof. Owen on the Atlas, Axis, and Subvertebral 



in front with the wedge-shaped cortical part of the body of the 

 atlas c a, ex. The lower part of the second wedge-bone is pro- 

 duced into a short spine. 



There is a distinct cartilaginous rudiment of a rib (pleura- 

 pophysis, pi, 1) attached to the diapophysis (transverse process 

 from the neural arch) of the atlas, and another from that of the 

 fourth vertebra (pi, 4). The first ossified pleurapophysis (pi, 5) 

 occurs on the fifth vertebra ; and beneath the diapophysis support- 

 ing this rib, there is a cartilaginous rudiment of a parapophysis 

 ( p) ; the same is still more plainly seen in the sixth cervical verte- 

 bra, but the heads of the pleurapophyses are simple. 



The odontoid piece (c a) is convex from side to side, concave 

 from above downwards ; is firmly attached to the fore-part of the 

 body of the axis and to the second wedge-bone, but with visible 

 traces of the suture remaining. The lower part of the body of 

 the axis is carinate, but not produced into a spine. A third sepa- 

 rate ossification in the capsule of the notochord (c S, e x) is wedged 

 into the inferior interspace between the axis and third vertebra ; 

 and similar but successively smaller wedge-bones (c 4< e x, c 5 e x) 

 are articulated between the fourth and fifth, fifth and sixth, and 

 also between the sixth and seventh vertebrse in the Amblyrhyn- 

 chus. If the odontoid process be interpreted as the homologue 

 of the anterior anchylosed body of the atlas in the Plesiosaurus, 

 the first wedge-bone will stand in the same relation to it as the 

 second wedge-bone does to the axis, the third to the third verte- 

 bra, the fourth to the fourth, and so on. These wedge-bones 

 are plainly the special homologues of the c subvertebral wedge- 

 bones ' discovered by Sir P. Egerton in the Ichthyosaurus; but 

 their general homology is open to two interpretations. They 

 are, no doubt, autogenous ossifications in the under part of the 

 capsule of the notochord ; but, as such, may be interpreted either 

 as parts of the cortical layer of the centrum of their respective 

 vertebrse, or as rudimentary hsemapophyses and imperforate ho- 

 motypes in the neck of the hsemal arches and spines in the tail*. 

 According to the latter view, what has usually been regarded as 

 the centrum or body of the atlas in Saurians, Chelonians, and the 

 higher Vertebrata would be the hsemapophysis of that vertebra ; 

 and the odontoid process the true centrum. But against this 

 view militates the constant relation of the inferior wedge-shaped 

 bone of the atlas in Saurians, Chelonians and higher Vertebrates 

 to the neurapophyses, as immediately supporting them and com- 

 pleting with them the neural arch. The obvious serial homology, 

 also, of that lower part of the atlas (c a, e x) with the basi-occipital 

 and basi-sphenoid leads me to conclude that, like them, it is the 



* In the Bayrus, in fact, the corresponding ossification of the noto- 

 chordal capsule is actually perforated by the aorta. 



