34 Field Columbian Museum — Geology, Vol. II. 



have a free, irregularly trapezoidal surface, the longer, cephalic bor- 

 der articulating with the atlantal neurapophysis, the caudal, oblique 

 border with the axial intercentrum, the ventrocaudal angle with the 

 axial intercentrum, and the ventral part of the caudal border with the 

 axial rib, its dorsal angle touching the axial neurapophysis. The 

 ventral side of the body unites suturally with the intercentra. On the 

 dorsal surface there is a narrow, free space, which helps form the 

 floor of the neural canal. 



The lateral pieces or neurapophyses are composed of a large body 

 for sutural union with the other elements, and a small, laminar, 

 dorsal projection, which is free. The body is irregularly five-sided, 

 the external free surface convex dorso-ventrally. The cephalic sur- 

 face is smooth and concave, forming the lateral rim of the condylar 

 cavity. Ventrally a small articular surface is for union with the 

 condylar rim of the atlantal intercentrum; the border nearly parallel 

 with the upper. Caudad, there is a small articular surface for union 

 with the arch of the axis; the long border between these two joins the 

 lateral surface of the odontoid; the dorsal surface sends a flat pro- 

 jection backward to touch, or nearly touch the small, vestigial 

 prezygapophysis of the axis; this lamina is continued into a small, flat 

 process, the vestige of the neural lamina. These neurapophyses are 

 peculiar in articulating at their base for a short distance with the 

 neurapophyses of the axis, doubtlessly corresponding to the articular 

 surfaces between the body of the axis and the lateral masses of the 

 atlas in the mammalian vertebrae. It is very plainly evident, there- 

 fore, that the name zygapophysis, when applied to this articulation in 

 the mammalian axis and atlas is incorrect— the real zygapophyses are 

 completely lost. 



The atlas, as a whole, is of a primitive and generalized character, 

 in that the neurapophyses are, for the most part, borne by their own 

 centrum, and the atlantal arch only in small part by the atlantal 

 intercentrum. 



The axis has its anterior surface flattened for sutural union with 

 the odontoid. On the cephalic ventral part is received the axial 

 intercentrum, the lines of the union reaching midway of the body on 

 the ventral side. The pit for the reception of the rib is very large and 

 deep, and is formed in part by the odontoid and intercentrum. The 

 pedicles of the arch are stouter and broader than in the following 

 vertebrae, articulating in front with the lateral pieces of the atlas, 

 and, to a slight extent, with the odontoid. The prezygapophyses 

 are represented by a small tubercle, approximated to the flattened 

 posterior process of the lateral atlantal pieces. Back of this there is 



