PROFESSOR HUXLEY ON THE OSTEOLOGY OF THE GENUS GLYPTODON. 59 
The Atlas . — Of this bone the mutilated right half is represented in Plate VII. fig. 1, 
giving the anterior, and fig. 2 the posterior aspect of the fragment. 
The specimen exhibits rather more than the right half of the lower arch, and rather 
less than the corresponding portion of the upper arch of the bone. The right lateral 
mass, with its anterior and posterior articular facets, is almost entire, but the transverse 
process is broken off close to its origin. The inferior arch is a solid bar of bone with a 
straight upper and a convex lower contour ; and somewhat thicker in the middle, both 
from above downwards and from before backwards, than at the sides. A section taken 
through the median plane of this part of the bone would have the shape of a spherical 
triangle; the lower or horizontal face convex, the anterior slightly concave, and the 
posterior and upper also concave. 
The middle of the posterior and upper face of the inferior arch presents an oval arti- 
cular facet (fig. 2, a) for the odontoid process of the axis, which, when entire, must have 
measured about 1-6 inch in width by 0-8 inch in antero-posterior length. It is slightly 
concave, both from before backwards and from side to side, and is bounded by a well- 
defined though narrow ridge. The outer end of this facet is half an inch distant from 
the inner and lower edge of the articular surface for the odontoid vertebra, upon the 
lateral mass of the atlas (fig. 2, b). This is a reniform surface with its inner and anterior 
side concave, while the outer and posterior aspect is convex. Its long axis is almost 
vertical, while the plane of its surface, which is a little concave both from above down- 
wards and from side to side, is directed obliquely inwards and forwards. Lines drawn 
through the shorter axes of the two articular facets would intersect one another at a 
point very slightly in front of the anterior margin of the inferior arch. The foramen 
for the vertebral artery is situated on the outer side of the facet, opposite the junction 
of its middle and upper thirds, and nearly on the same level as a tubercle for the trans- 
verse ligament, situated on the inner side. 
The foramen (fig. 2, c ) leads into a canal which passes directly forwards, widening 
as it goes, and traverses the root of the transverse process. In front of this it presents 
a large oblique aperture, by which, however, it does not terminate. Instead of ending, 
it makes an abrupt turn upwards through the substance of the superior arch of the atlas, 
parallel with, and equidistant from, the anterior and posterior margins of that part, and 
ends by an oblique aperture in the outer part of the roof of the cavity of the atlas, and 
nearer the occipital than the odontoid edge. The upper face of the lateral mass of the 
atlas presents an elongated, irregular, transverse aperture, which communicates with the 
canal, and from the anterior and posterior margins of which broad and shallow grooves 
are continued. 
The articular surface for the occipital condyle upon the anterior face of the lateral mass 
of the atlas (fig. 1) is much more concave from above downwards than that just described ; 
and as it is neither concave nor convex from side to side, the surface may be regarded 
as a segment of a hollow cylinder, answering to rather less than half the circumference 
of such a figure. When the inferior arch of this atlas is made horizontal, this articular 
