BY W. J. McKAY. 921 



tubercle is a small pedicle of bone ; this runs forward and down- 

 wards, and is regarded by Owen as representing a parapophysis. 



Hypapophysis : The hypapophysis springs from the whole of 

 the inferior edge of the centrum. It runs downwards and back- 

 wards. Its anterior edge is longer than its posterior, and slightly 

 concave from above downwards. The external sides are rounded, 

 and give attachment to the depressores costarum and sub vertebral 

 rectus. 



Atlas : The atlas presents as in all vertebrates the most con- 

 siderable modifications. The anterior face has three articular 

 facets, while occupying the region of the centrum is the elliptical 

 prominence of the anterior extremity of the odontoid process. 

 Anteriorly and inferiorly this latter slopes rapidly downwards and 

 backwards, while superiorly it extends upwards and backwards 

 for a short distance only. On either side it meets the lateral facets 

 of the atlas at an acute angle. These facets, which represent the 

 articular surfaces of the lateral masses, spiead out from the 

 odontoid process like wings ; they are triangular (the apex being 

 superior) and concave. Each plate is placed so that its surface 

 slopes downwards, backwards, and inwards, to meet the odontoid 

 process, and thus, as mentioned above, an acute angle is formed on 

 either side. The facets articulate with the exoccipital moieties 

 of the trefoil condyle of the skull. The third face is the superior 

 surface of the autogenous hypapophyseal portion of the bone. It 

 is pentagonal in shape, the apex being in front. The surface is 

 concave and lies anterior though inferior to the odontoid process, 

 which it meets behind, making with it an angle of 60°, The an_ 

 terior portion of this face presents a distinct ridge, which enables 

 the occipital condyle to hook on most effectually. The neural arch 

 is formed by two curved laminae of bone running up to meet in the 

 mid-line, the neural canal being wider, though less high, than in 

 other regions. The neural spine is absent, a slight ridge taking its 

 place. There is no prezygapophysis or zygosphene; the anterior 

 superior edge being, however, in close relation to the posterior 

 superior edge of the exoccipitals. A small prezygapophysis is 



