34 FIELD COLUMBIAN MusEUM—-GEOLOoGY, 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 datera/ 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 
pecuhar 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 ax7s 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 1s 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 
vertebre, 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 
