The Skeleton 53 
processes that bear the laterally and ventrally 
facing postzygapophyses (Fig. 17, 6). 
The atlas, as in other vertebrates, is highly 
specialized. It consists (Fig. 17), even in the adult 
animal, six feet or more in length, of four distinct 
portions, a ventral (4), a dorsal (1), and two lateral 
(2) parts. The ventral portion is relatively more 
massive than in most animals; its anterior surface 
is concave and forms the main part of the articular 
surface for the occipital condyle of the skull. Its 
postero-dorsal surface articulates with the odontoid 
process of the axis. On its postero-lateral surfaces 
are the facets for articulation with the first ribs, 
which, unlike the other cervical ribs, have but one 
articular surface. Articulating dorsally with this 
ventral element of the atlas are the two rather 
heavy lateral elements which form the neural 
arch. Anteriorly they form the lateral parts of the 
articular surface for the condyle and dorsally they 
unite for a short distance with each other. Pro- 
jecting ventrally from the posterior part of their 
dorsal portion are the small postzygapophyses. 
Ventrally and laterally they articulate with the 
odontoid process (Fig. 17, 3). Projecting dorsad and 
cephalad from the dorsal surface of these lateral 
elements is the dorsal element of the atlas (Fig. 
17, I), the pro-atlas, which may not be properly 
a part of the vertebral column at all, since it is said 
to be merely a membrane bone. Gadow says it 
is the detached neural spine of the atlas. It is 
