388 



ME. ST. GEOEGE MIVAET ON THE 



presents a crescentic, transversely extended concavity, as it were a cup, with a consider- 

 able median dorsal portion cut away for the reception of the odontoid process of the axis 

 vertebra (fig. 2, ac). 



ATLAS VEETEBEA (natural size). 

 Pig. 2. Tig. 3. Pig. 4. 



Pig. 5. 



Pig. 6. 



Aspects. 



Pig. 2, preaxial ; 3, postaxial ; 4, lateral ; 5, dorsal ; 6, ventral ; 7, preaxial of immature bone. 



ac, Preaxial articular surface of centrum ; ac', portion of such surface formed by neural lamina ; d, diapophysis or tubercular 



process ; !:p, hyperapophysis ; hi/, hypapophysis ; pc, postaiial articular surface of centrum ; pc' lateral prolongation of the same 



surface ; v, canal for vertebral artery (the spiculum of bone which encloses this is seen to be wanting on one side) ; x, a rounded 



tubercle representing a lateral hypapophysial process ; z, postzygapopbysis. 



The dorsal margin of this preaxial surface forms thus a strongly concave line ; and its 

 ventral margin is bounded by a longer line of generally similar curvature (the horns of 

 the crescentic articular surface being as broad as its middle) ; but the median portion of 

 the ventral margin may occasionally be slightly produced (as in the specimen 1362 b), 

 so as to cause that margin to present a rounded prominence. 



The ■postasial surface, which is applied to the centrum of the axis ventrad of the odon- 

 toid process, presents a crescentic transversely extended articular surface (fig. 3,pc,j) d). 

 This surface is gently convex dorso-ventrally, but straight and flat transversely. Its 

 upper margin is strongly concave, and is produced preaxially on each side, the postaxial 

 articular surface of the quasi-body thus extending on each side somewhat on to the dorsal 

 (neural) surface of the same part, and there articulating with the sides of the ventral 

 surface of the root of the odontoid process. 



The ventral margin of the postaxial surface of the atlas is more or less convex, with 

 its median portion drawn out into a postaxially projecting hypapophysial process (figs. 3, 

 4, & 6, hy). 



The neural surface of the quasi-body is gently concave transversely, with a marked 

 pit (fig. 5) in its midst, no doubt for a ligament proceeding dorsad to the pit at the root 

 of the odontoid process on its ventral aspect. 



