Wedge Bones in the Plesiosaurus. 223 



inferior cortical part of the body of its vertebra. In the cranial 

 vertebrae this usually depressed and expanded cortical part, exem- 

 plified by the basi-occipital, basi-sphenoid,presphenoid and vomer, 

 is the sole representative of the centrum of such vertebrae : in the 

 atlas the odontoid process would represent the central part also 

 of the body of the vertebra, but detached from the cortical part. 



The following facts, however, appear to oppose themselves to 

 the determination of the ' odontoid ' as the central part of the 

 body of the atlas. 



In the great Australian Skink {Cyclodus gig as) the second 

 wedge-bone (fig. 5, o d, ex), which is developed into a long spine 



Cervical vertebrae, Cyclodus gigas. 



and is anchylosed to the under part of the odontoid piece, is not 

 the only inferior or haemal spine of the axis vertebra; but a 

 second broader and longer spine (c x, e x) is developed from the 

 under part of the proper body of the axis. And the fact of 

 the absence of any suture between this spine and the body of 

 the axis is not enough to support the conclusion that it is a mere 

 excess of development of the under part of the body of the axis 

 and no true homotype of the inferior spines or wedge-bones ; be- 

 cause, besides the anchylosis of the preceding spine (o d } e x) 

 with the odontoid piece, the fourth spine (c 3, e x) is equally 

 anchylosed with or developed from the whole under part of the 

 third cervical vertebra of the Cyclodus ; and the fifth spine (c 4, ex) 

 is a similarly continuous process from the under part of the fourth 

 vertebra. As the odontoid piece and its spine are completely 

 anchylosed with the axis, this vertebra presents the anomalous 

 structure of one neural spine and two consecutive haemal spines. 

 The above-described structure of the anterior vertebrse of the 



